For things like OBD monitoring I sort of prefer my own Android device. There are already bluetooth based dongles for reading OBD. Car manufacturers would probably lock their OS down hopelessly.
Yeah I've seen one, pretty nifty (but messy with all the wiring). The idea would be that the manufacturers don't lock it down to allow outside developers to bring value to the platform and utilize additional APIs. I made a mockup of how it would look like: http://s13.postimage.org/8bt3a7kdj/Car_Droid.jpg