I've looked for the same thing for quite a while. Here is probably the most useful link I've found:
KW1281 Live Diagnosis with Raspberry Pi
The author reverse engineers a piece of software written for Windows by looking at the byte streams going between his computer and his car's computer. While the technical details are interested, he also provides snippets of source code that can be used.
His final conclusion is that the Raspberry Pi is not able to read from the OBD-II interface of a car while the engine is running. His reasoning is buggy serial drivers resulting in bad timing.
Here's another one that may provide a solution eventually, but mostly covers the hardware side for now:
Raspberry Pi Car PC Project
The author requested a free STN1110 chip and plans on making a custom board to communicate with his car. If you do a search on the Raspberry Pi boards you will find a couple other people working on the same thing. I was even able to find a person working on a marketable version of such a board:
OBD-II STN1110
The creator did not provide a website for ordering such a board that I'm aware of, but seems to offer them if you send a private message on the form.
There is also an android app called Torque that is available. I haven't managed to find source for it however.
Some more potentially useful links:
pyOBD - Open source (GPL) program written in python that interfaces with OBD-II. If that's not a perfect match for the Raspberry Pi, I don't know what is.
openOBD: OBD-II Scan Tool - An abandoned C++ based OBD-II program for both Windows and Linux based systems.
Edit 7/28/2013
There was a blog post on RaspberryPi.org today with some more good information:
Here's a link to the blog: Carputers – some ideas to get you started
A link to a forum post of a guy who put a PI in his F150: My Pi Carputer in my truck.
A link to his github source.
Another project with OBD and GPS: OBD GPS Logger
Software written for the above that simulates OBD: OBDSim
I have a feeling that if I don't start working on this soon there wont be anything left to develop.