I've always wanted to get into micro controllers
I know you can buy, from SAE, the book on the OBDII protocols and how to interface - probably something necessary if you want to plug into the diagnostics port and not tap a bunch of misc. wires.
Ladyada makes an arduino clone - the boarduino -
http://www.ladyada.net/make/boarduino/ and is sold as an assembly yourself kit for ~$17
Just food for thought. I don't have time now, but if something gets going - I'll throw myself into the project (throwing myself deep into projects is both an asset and a weakness of my personality
)