What happens if I use less than 5v and something like this http://www.sparkfun.com/products/341 ?

Bro, It's only 3.7v. But you have the right idea. Do some battery research. Didn't you say that you needed more than 250mA? Just ger a 9v NiMh  rechargeable battery from Radio Shack or Office Depot along with an LM 7805 and be done with it. Do that, and I'm sure anybody will be happy to help you wire it. And there are already tons of tutorials about it anyway.

A zener diode seems like the cheapest and easiest solution, but is it reliable? Is there a benefit to using something like this http://www.sparkfun.com/products/107 ?

If you scroll down to page 3 of the data sheet it says that it needs 7v minimum for input. I mentioned that earlier.

A 5v zener diode should be fine. Just check the data sheet.

An easy and cheap way would be to use a zener diode rated at 5v. The problem with using resistors in a voltage divider circuit is that as the battery drains, your 5v becomes much less. It all depends on how much current you need or that may not be a problem. You only need 3.3v minimum right? The low drop out regulators are expensive (~$7). You may want to consider getting something like a rechargeable 9v NiMH battery. They're cheap, available at office supply and electronics stores, and throw away alkaline batteries are crap. Then you would be able to use a LM7805 voltage regulator. The LM7805 is cheap, but you need to input at least 7v.

A TIP120 is much less likely to burn up than your PN2222.

You need to divide Val_1 and Val_2 by 2 in order to get inches. If you want centimeters, you have to then multiply that result by 2.54.
http://www.arduino.cc/playground/Main/MaxSonar

Also your sensors are almost certainly interfering with each other. Can't you just use one sensor? If not you need to daisy chain them or learn how to trigger them. There are instructions on how to trigger them earlier in this thread.

http://www.instructables.com/id/Getting-started-with-the-Maxbotix-sonar-sensor-q/

very good sketch! I think you should try to use the pwm mode of the maxbotix sensor. It's not much different from what you are doing now. You don't have to daisy chain it either. They won't interfere. They are on different sides If the door right? I'll walk you through it. Just post some code using the links I gave you. If you don't understand I'll help you. I know it's frustrating, but I'm here to help. We'll make it happen.  

the maxbotix sensors will work, and if they are on opposite sides of the doorway they won't interfere with each each other. check the links from my last post. Get them wired up and write some code. Post a schematic and your code here, and I'll check back to help you.

check out this tutorial:
http://www.jeremyblum.com/2011/01/24/arduino-tutorial-4-analog-inputs/

What is the mosfet for? You can make a voltage divider circuit with a couple resistors if you want 3.1 volts to go to the sensor, but it'll also just use the 5 volts from the arduino just fine (see the data sheet). Try that, and see what happens. There are also much simpler ways to write your code. Was that code written for some other function that you adapted for this purpose?
This string is about a ping sensor and fading an led, but it should give you some ideas: http://arduino.cc/forum/index.php/topic,86560.0.html

It depends on what mode you want to connect them.
Read through the Product Documentation links from this link: http://www.maxbotix.com/products/MB1030.htm

And another:  http://www.arduino.cc/playground/Main/MaxSonar

If you just want to detect the presence of a person, passive infrared sensors are made just for that. Ultrasonic sensors are made for sensing distance. What exactly are you trying to accomplish? If you are more specific, we could be of more help.

Awesome! That's great news. I'm most interested in human detection, but we're definitely on the same page. I have been getting some pretty accurate and precise readings with my maxbotix sensors using the crap (code) that I came up with recently, but there's always room for improvement. It's not in a library, but it works. For my purposes I don't even need to be that accurate, but I need it to be reliable. I have several ultrasonic sensors that I've been messing with, but I always get these stray readings. One or two are fine within a half second, but more than that mucks up my purpose. I will try to implement your library with my sensors (maxbotix) soon, and hopefully get some quality results.

I'm not sure what you mean by protection fuses. Sounds interesting, though.  I think at one time in my schooling I learned that with PIC chips, etc. there is equipment that can read the machine code produced by the C or PIC Basic Pro compiler. Anyway, my project isn't a time machine or lotto number picker so I'm sure I'm safe hahaha.

http://www.arduino.cc/playground/Learning/OneWire
I would read through this again, assuming you have already. Next I would recheck all my connections. Sometimes it's best to just undo everything and rebuild.

After reading some of the forums here concerning licenses, copyrights, etc., I'm a little confused. If one wished to make a commercial project using the ATmega 328 or any other Arduino compatible chip and didn't wish to have your project ripped off, can't you just not share the code? I know it's more complicated than that. I guess my question is: Can someone with the right equipment read and/or reproduce your code if they have your programmed chip in hand?