This thread has made me want to try something with ODB-II as well. So I've requested some sample STN1110 ICs from here: STN1110 - Multiprotocol OBD Interpreter IC - Low-Cost, High-Performance
If they ship me one I'll begin working on a datalogger/diagnostic tool I've been wanting to make for a while now.
The STN1110 chip supports all the OBD-II protocols. If you want something a little more user-friendly (and a little more expensive) you can also get the ODB-II UART board from Sparkfun (SparkFun OBD-II UART - WIG-09555 - SparkFun Electronics) which also uses the STN1110.
Or you could build your own version of the Sparkfun board using their schematics.