I'd like to share my progress on a portable Raspberry Pi system with a 3.5" display and a 4-wire resistive touchscreen. The interface is all through I2C, leaving the USB ports open. Everything runs off of a single 12V power supply, with 12V going directly to the display and 5V to the Pi through a step-down buck converter. Drivers are fully implemented as kernel modules and the input is configured and calibrated through Xorg and work on the default Raspbian.
There are still some issues to work out, and I have to get it all packaged in one secure setup, but all in all it's coming along quite well. If there is enough interest, I can post instructions.
You can see pictures of the work as it went along here
And a video of it in action here: