I was inspired to post this by ART's awesome gpsp.
All (almost all) cars built after 1996 have an on board diagnostics port that conforms to a standard called OBD II. These are the interfaces car shops use to interact with your car when the check engine light comes on and whatnot. Certain companies have created interfaces with these ports which can give you access to data about your car (temperatures, pressures, rpm, gallon's per hour, etc, etc.)
Most, if not all of the interfaces made to connect car to pc or pda are serial port in nature (ART is paving the way with this type of connection through the headphone remote). So, I propose a similar interface for the PSP. The necessary steps are:
-serial interface to the psp...use a similar method to the gps mouse
-no problem with power...the obd II port puts out its own power (12V probably?)
-obd II is an open protocol/spec with plenty of documentation and open source code to interpret the outputs from it.
-build an interface that displays several of the variables transmitted through the port on the psp...could be done with lua, python, or any other established method.
some links of interest
http://www.scangauge.com/ ...a comercial device to give and idea of what info can be had from the port
http://www.obdii.com/ ...the standard
http://www.digimoto.com/ ...and interface from car to computer/pda