I was introduced to the Torque App for the Droid (and soon Blackberry) market today. In a nutshell, it is an OBD2 reader for your phone! Plugged it into the A4 today, not only did it read my engine code fault but I was able to reset the light....eat your heart out VAG COM

It also uses the OBD link to give you a real time 'dashboard' of whatever you want displayed (boost, speed, coolant temp, fuel pressure, engine load, and on and on ).
The free version of the app does most things, the paid version is only $5. A bluetooth OBD2 adapter (oh ya, its wireless baby) can be had for as little as $20 or so I've heard. The adapter I tried today was a really nice one that cost ~$100.