So then this begs the question, what diode should I be using?  Actually, I am trying to figure out how to hook it up.  Don't I need a relay in there instead of a diode?

Space is a concern.  But you just gave me an idea I can't believe I didn't think of.  A pocket bike battery is also lead acid.  This should be enough.

http://www.batteryspec.com/cgi-bin/cart.cgi?action=link&product=300G95

Thanks, your idea will work with that battery.

So you are saying have the arduino be powered by which ever source and control which source is powering it?  That sounds like it could fail at some point and the arduino would need powered at all times.

Yes, I will have a master switch to shutoff the project.  I am hoping to keep the project component separate from the power, the power supplies (car and lipo) only supply power.

How about a little more detail.  The project will run off the lipo most of the time.  Most of the time I will just shut it off while the car is running (not legal to have this on while driving on public roads).  So while I am driving why not have the car charge the battery is what I am thinking.  However, if parked at a show and the car is running I figure why waste the lipo battery when the alternator is running also.

I am going to need to power an arduino and several of these sparkfun led strips.  2 strips would almost be 4 amps.

I was thinking about using a Traxxis 7.4v 12800mAh R/C car battery.  Those can handle the large loads.  I am not an expert in power supplies.  I've been researching what I can do and I have hit a wall.  I would like the car power system to charge the battery while it is running and the project use the power from the car.  Then when the car isn't running to use the battery's power. 

i am not sure how to accomplish this.  I see some obstacles.  One, need to regulate the car voltage as it can spike.  Two, how to charge the battery while using the car power system.  I would obviously used a switched power line in the car, however, to protect form accidentally draining the car battery if it isn't running I could use a voltage switch that disconnects the car battery form my project.

My backup plan is to just have a single switch that when off the car power is connected to a charger to charge the lipo battery an the lipo is no powering the project. When on the switch will cut off the car's power system completely and connect the lipo to the project.  i would rather have an automatic way instead if possible.

I found this but I am not sure if that can be used for what I want.

http://www.sparkfun.com/products/10312

that as well.

Alright, I got the RBBB.  Same exact issue.  This means that somehow the Mac side is rejecting my FTDI cable.

Yes, I tried doing just a simple sketch.  Like I said, I uninstalled the arduino software and all extra libraries (in the documents fold on both mac and windows).  I then just tried the blink led sketch, on both windows and mac.  It uploads fine on windows, but on the mac side I get the error.

The GPS shield is not using pins 0 and 1 when it is on the arudino as I can upload fine on windows.

Yes, I did search.  But as you said it has come up a lot lately so there are a lot of searched to sift through.  None of what I read seems to be the appropriate answer.  Most are saying the cable/ftdi connection is bad or the wrong board was selected.  If either of those were the case it wouldn't work on the windows side.

So, the conclusion then is somehow the Mac side has decided to not like the FTDI cable anymore.  I have an RBBB arduino on the way, I will have to try with that when I get it.  I have this cable: http://www.sparkfun.com/products/9718

I did notice when double checking the board to select in the software the two Arduino Pro entries are the same description.  They both say the 128 version.  No 328 version listed.  I tried all possible 328 boards.  As I said, it worked a couple weeks ago but not now.  I don't remember what board was selected a couple weeks ago.

http://arduino.cc/en/Main/ArduinoBoardPro

"new module" is new GPS module to replace the defective unit.  I bought the GPS shield and EM-406 GPS module form sparkfun.  The antenna wasn't working on the module I received.  The module was communicating with the arduino over serial but not getting a fix.  After talking to sparkfun support we found it was a defective GPS module.  So I had it replace.  I didn't change the code or install any new libraries for the Arduino. 

Obviously it worked on my Mac before I sent in the defective unit.  After hooking up the new unit I got that error.  I booted to the Windows side of my Mac and also using arduino 0022 it worked fine.  So it isn't the FTDI cable and it probably isn't the hardware.  Somehow the software changed.

I tried uninstalling the arduino software and removing any third party libraries I had.  I then reinstalled the arduino software and tried the basic blink led sketch.  I still got the error.

I am confused.  A couple weeks ago I got an arduino pro 328 and the gps shield.  I am using arduino 0022.  It worked fine but the GPSr was defective.  I got the new module and now I get
avrdude: stk500_getsync(): not in sync: resp=0x0d
avrdude: stk500_recv(): programmer is not responding

I booted to windows and it works fine there.  Why would it fail in windows?

If you need a shield then it is kinda tough.  If you don't there are several options.  The RBBB mentioned before.  Maker Shed has something similar.
http://www.makershed.com/ProductDetails.asp?ProductCode=MKSB012

I use this for simple projects
http://www.makershed.com/ProductDetails.asp?ProductCode=MKSKL4

How close is it going to follow? Would there be a delay?

On a good day GPS is accurate up to 12 feet.  On average with a day that isn't too cloudy with direct line of sight to the sky 15-20 feet is typical.

I wonder if some type of radio signal on the leader and a signal strength detector on on follower.  It would be like those automatic solar panels that use two photo sensors.  It keeps rotating until the sensors read the same amount to keep the panel looking directly at the sun.

Does the SD card class make a difference or is it the hardware most Arduino users use?  Could we get full speed from a class 10 card?

Thanks for the info.  I see two routes to take.  Cellular is one, the other is APRS and ham radio.
http://trackuino.blogspot.com/

It isn't hard to get a ham radio license.  And it is a standard data format.  There are several sites you can use to track that data onto google maps.
http://aprs.fi/?call=a%2FN0KFB&mt=roadmap&z=14&timerange=21600

What service/plan is your unit going to use, if you don't mind me asking?
i ask because I recently asked a GSM related question in project guidance
http://arduino.cc/forum/index.php/topic,51823.0.html

I have been doing something similar over the last year for my motorcycle.
http://arduino.cc/forum/index.php/topic,8381.0.html

I started it last spring.  But then the weather got warm and I would rather spend my time on the motorcycle   I was also delayed because customs held my bliptronic leds for a couple of months.

I have gone through 4 different designs because I keep adding features.  I'll give you a quick summary of what I have done and why.
My original plan was to make some accent LEDs that I can control and make custom effects.  I wanted to control them remotely and I wanted the ability to add effects later on easily.  I found a 4 button remote.  I will have a microsd card holding animation information.  Some animations have to be programmed - anything that has random numbers.  The microsd card will will contain frame based effects. 

I started with a tlc5947 led driver chip as that is the first thing I came across in my research.  The first video in that post is me making sure I can control the chip.  Those LEDs are once I had around and they are a pain to work with.  I found a led strip on ebay that takes 12v (perfect for a car or bike) that could be cut and used like individual LEDs with the tlc chip.  I got that all working fine.

Then I saw the Bliptronic pixel LEDs.  I really liked that idea over the tlc5947 because it would be a lot less wiring.  In a car, you can hide wires in door panels and such.  On a motorcycle, not so much.  So I picked up some of those.  They work great but I ran into a problem.  Since you have to manually control the PWM cycle that takes up processing power.  Some of my more complex animations affected that causing the LEDs to flicker. 

Luckily, the next week Sparkfun came out with their addressable LED strip.  The chip they use has it's own PWM cycle.  That is a super easy LED strip to work with.

Right now I am in the process of converting the code to a Netduino Plus.  The Netduino Plus has built in ethernet and microsd.  This gives me more I/O pins for future expansion.  Plus the fact that I can do line by line debugging makes it easy to code. I am going to use a small wifi router connected to the ethernet port and control it with my ipod touch instead of the remote.

I am still going to finish the project with the Arduino as my sister wants the basic setup on her bike.  One idea for future expansion is to use xbee or some other wireless communication between my bike and my sister's bike to have in sync animations when our bikes are next ot each other.

When I ordered the Bliptronic LEDs I also grabbed the audio analyzer shield.  I will add some modes where the LEDs are controlled by music.

Finally, FYI, the US government definition of amber warning lights is actually orange.  100% red, 50% green.  I was going to use my LEDs as running lights (anything to make a motorcycle more visible) but running lights are only legal on trucks and emergency vehicles.