There is actually a company called Tunaverse (http://tunaverse.com) that is doing/working on this. You can stream any of your media files from your home computer to your phone or any other device with an internet connection.
I have no connection to this company other than I met one of the founders a year ago and he was telling me what they were working on. I haven't talked to him since and don't know how far along they have come, but their website is still up (just checked) and looks like they have the client running for Windows Mobile devices at this point.
The hard part here is clearly not getting data into whatever audio player is available (ie. car radio), it's getting ubiquitous wireless in each of these devices, so that one does not need to carry around and plug in a device like the GPhone. Does anyone know if AT&T or any other wireless providers have any plans to go into this market? Obviously, tying a device to a particular network sucks because it locks you into that network, but I would be much more likely to pay for a monthly data plan if I could get a car radio that used the same connection to stream music!
I imagine the problem here might be licensing, but I'm not really knowledgeable enough to know.
I have no connection to this company other than I met one of the founders a year ago and he was telling me what they were working on. I haven't talked to him since and don't know how far along they have come, but their website is still up (just checked) and looks like they have the client running for Windows Mobile devices at this point.