Maybe this late, but I just started making my OBDuino.
I'm using the device from dealextreme:
It contains the elm327 with all necessary peripherie and an FT232BL.
The FT232 chip ist just an "virtual COM Port to USB"-Chip.
So you just have to wire your arduino to the RX/TX Pins of the FT232 and then the OBDduino will work!! Oh don't forget to connect ground
Pictures + Videos coming soon...