I just finished building my own OBDII port interface from a kit purchased at
http://obddiagnostics.com/ for $55. I also had to purchase about $15 worth of electronics and spend an hour or so soldering it together. Much to my amazment... it works! They also provide free downloadable software that can read and reset engine error codes, do high-speed data logging (3 or 4 samples per second), a cool-looking gage display and several other features.
My CarPC is still in the assembly state (on my work bench) so I used a 15' network wire as the serial cable. I was going to shorten it when the PC gets installed... but it's so cool, I think I leave it long enough to reach other cars... you never know when a friend might need an engine code reset!