Shutdown controler for PDA

I have a working carputer and it works slick. A family member commented that they would like the GPS navigation system in their car, so I thought whats the cheapest way to get iGuidance in a car? We don't need MP3 playing ability here. I considered a old 133 mhz laptop as well as a few other things, and what I ended up deciding on was a PDA. You can get them pocket PCs for like $50 off of ebay, a GPSr for about $80, and a decent amount of SD card storage for about $20. So considering I get a PDA, put iguidance on it, and have it fiberglassed into the dash at some spot, my last question is:

What would be an effective shutdown controler for a PDA based carputer? It needs to have the ability to turn on and off, and off should be achieved without taking out its battery.

Keep in mind anything ghettorigged is fine, what I need is cheap, not nessesairly state of the art.

-OR- could someone recomend a hardware solution that would run IG and have working gps hardware for less than $140?

Alright, well I brainstormed shutdown; if device unpowered (car off=inverter off=transformer off=running off of batteries) for more than 1 min then standby, otherwise remain powered.

All I need is a creative way to turn it on now :P