I was talking to a guy at Best Buy about setting this up for my car. It looks like you can buy a kit or buy an ipod-ready car stereo.
I think it would be an interesting idea to talk about in-car connectivity for portable music devices.
I've had the opportunity to test several cars here in Detroit that run the gammut in connectivity options. Both Ford and GM are planning on including some type of iPod connectivity options by 2008. Some will include an interface that shows playlists, others will just allow for an external cable. If Bluetooth players hit the market, expect the manufacturers to include them in their list of discoverable devices their in-car software supports. DaimlerChrysler is introducing a 30 gigabyte "MyGig" personal hard drive in selected models for 2007 that will not only store music and playlists, but will allow calendar and photos.
Let me know if I can help with any research, as I work in the auto industry.