Check your baud rate.

That's very cool. It's something like what they have on those GSM base station where they can control the antenna direction, frequency, and other parameters from a web interface.  

As for "working out of the box", is there a way to test this?  If I restore the default values, what do I need to do to see if they are communicating to each other?

I assume you have the XBee carrier board (a serial or USB to PC). You can set the Coordinator destination address to the Router/End Device's 64-bit IEEE address number (consists of SH & SL in the XCTU). This can be done in the XCTU program. Then connect the Router/End Device to a PC. Use a terminal program to send out test data continuously (1 second maybe). Short the RX and TX pin of the coordinator. You should be able to see replica of what you send on the terminal program.

By using this method, you eliminate the mistake that you might have made on a microcontroller (coding, wiring, etc). Once, this is okay, you can move on to incorporate the Arduino into the setup.

Again, I understand the RX/TX thing, but I'm not sure I understand what you mean about the baud rate.  I know that both Xbee's need to have the same baud rate... was there anything else you were referring to?

The baud rate refers to the serial interface baud rate between the Xbee to the host (Arduino, PC). So, both the host and the XBee must have the same baud rate. You can have many XBee, but not all of them must have the same baud rate. Communication between XBee modules is fixed at 250kbps.

Here's a link to common mistake when configuring an XBee:
Common XBee Mistakes

My Address = FFFE (for some reason this changes on its own from time to time - help?)

16-bit address is dynamic address which is assigned by the Coordinator. Don't touch this.

xbee 2 = coordinator AT, PanID = 1616, My Address = 0 (this one DOESN'T change), Destination Low = 0, baud rate = 3/9600/default, Channel = 10

The channel should be same with the other Xbee in the network.

It should work out of the box actually without touching anything actually. I used them at lot at work and only change parameters for optimization and customization based on the applications.

Just ensure simple mistake like RX/TX pin swap and baud rate is not present, then you should be fine.

It look bad on Chrome too. The FAQ, Blog, Forum and Playground buttons cannot be seen unless being moused over.

the voltage regulator uses up unnecessary power

You need a voltage regulator that is low in quiescent current else it might eat up ore current than the MCU itself. Some LDO are very low in quiescent current and you can also opt for switching based regulator. But it also depends how long you want your system to run without permanent power.

Would a 555 timer be able to do this, or an RTC?

You can use an RTC because they consume very low current. You can look at jcl's solution, he is using an RTC and some very low quiescent LDO. The RTC time stamp can be used as your sensor data time stamp too. Besides RTC, you can also use external low power crystal provided you have the necessary MCU pins to drive it.  

You can also shut down the sensor when not using them. Some sensor has sleep mode, you can do that provided the sleep current is low enough for your application. Else you can just cut the supply to the sensor.

Cheers.

The way they described is as though Duemilanove is indeed made in China. It's rather hilarious but it would very bad if some people would fall for this.

Found this in one of the distributor (not official Arduino) webpage.
What a scam. Dueminalove is definitely no more in production.

The trick is to go for semi-glossy photo paper and avoid those expensive glossy papers. Any iron would do and use adequate design clearance in PCB design.

Code:

Which kind of 900 MHz and 868 Mhz radio are you talking about?

I'm referring to the ones from Digi, the XBee-Pro for 868 MHz version and XBee-Pro XSC for 900 MHz. For LOS, the 868 can reach 40 km with adequate antenna gain. Sparkfun carries them.

The newer XBee ZB Pro is 3.2 km (~ 2 mile) for LOS. You would need to use higher gain antenna though. If your region allows the use of 900 MHz and 868 MHz band radios, go for that instead.

IMHO, vendors like Dust Network, Synapse, and Greenpeak (just to name a few) are more eager to sell their products to larger cooperation. Furthermore, their modules are geared towards single chip (or single module) solution whereby the are promoting the use of the their software to configure and program. What I'm disappointed is at times, the sales guy always ask questions like how much is your consumption per year and how big is your company, etc. No doubt, the Synapse products are awesome products, I just wish that they have a wider distribution network like the XBee.  :-[

If you are able to just add in about $60, I would suggest getting the Rigol DS1052E. That is probably the best for money scope out there.

Just downloaded 0020 and tried on my own board version of Arduino. Uploaded the sketch like 0019 & 0018. I thought it would give a warning "You are using a clone version of Arduino!". Just joking.

It has to be LUFA!