As part of obdgpslogger
of the plugcomputer competition], I wanted a plugcomputer living in my car; powering on when the car powers on, logging data, and powering off when the ignition is cut.
I modified my plug to use a 12V-5V supply, and out of interest I took power readings in a few places. My writeup is here:http://icculus.org/obdgpslogger/sheevap5v/
I dunno how much interest that will be to most people, but there you have it.