Few ideas not yet mentioned for gyros:

• A gyroscopic presentation mouse could possibly be mounted in a fixed position. The receiver is USB, so that's how it would have to talk to the PDA. ($70 USD)
• A nav system with a built-in gyro -- normally only expensive nav systems have it, but geeks.com has an exceptionally cheap one. The down side is you have hack two devices; the Maemo app/driver, and the GPS unit. ($150 USD)
• A bluetooth gyroscope for aviation. This device must certainly have the precision required, but only practical for millionaires. ($1k USD)

Also, does the application you folks are discussing require OBD updates more frequently than ~0.03Hz? If so, that could be an issue. This video review of Carman/Elmscan 5 shows very sluggish realtime performance from the OBD interface (guages update once or twice a minute).