Hmm. IIRC, there are already some OBDII<->serial devices out there. I'd suggest going the "multiple-serial-ports" route, both for the ODBII *and* bluetooth (granted, serial emulation profile only).

There's already a serial<->bluetooth bridge from these guys (among others):

http://www.aircable.net/

I use their zaurus-specific dongle, but they've got a DB-9 version as well. Bit pricey, though.

-brendan