I am about to start work on this technique as well.
I have 2 older (pre OBD) engines that I want to monitor via an Android App called "Torque". One engine is running a Megasquirt EFI system and the other is using a Megajolt Ignition system.
The Megasquirt outputs CANbus Data and the Megajolt outputs Serial Data. I have already built a little nixie tube gadget to decode and display the serial data from the Megajolt (here - http://www.youtube.com/watch?v=nHiYCZkAq5o ) and am working on the CANBus Interface for the Megasquirt for some Thermocouples at the moment.
So my next phase is to get the Arduino outputting ODB data into a Sparkfun serial-bluetooth module so I can get Torque to read it as if it is coming from an OBD2 Car.
Im busy looking around to see what I can find in this area (which is how I found this thread). If I find anything useful, I'll post it back here.. If anyone else has any pointers on an OBD output library, let us know please. Im off to have a poke through the OBDuino Code :)