i have been thinking about this as well. It would be so easy to run the sound through the stereo and not have to invest in another video solution.
I have the first gen ipad with the apple cover that opens like a book. I have been thinking about rigging a string from the two headrests and then letting the ipad rest just above the rear air vents. Kind of jerry-rigged but i will only be using it on long trips. It wont be set up very often.