Fred Wilson posted today on the evolving model of using you mobile as your "primary" device, and this is the model his family uses for audio.
In fact, in my family we've been using this model for video for quite some time. Our Acura MDX has a DVD player and a screen for the kids to watch on their commute, and although for many years we primarily used the DVD player, more and more, we are now just plugging in an iPhone or iPad into the AUX jacks and playing video that way.
The key is this cable from Apple:
In fact, we've actually been using this to stream Netflix into the car as well. It works pretty well. Luckily I have a grandfathered unlimited AT&T data plan.
This same cable I carry with me when I travel as well. It plugs into most hotel TVs and provides a great way to use your mobile as your "primary."