It's been more than just thought of. A growing number of people are already doing it.
I have a Verizon Wireless XV6700 Windows Mobile 5 smartphone/pda. With an adapter, I plug the audio output into my car radio. With unlimited wireless broadband EVDO Internet access, I listen to Internet radio stations in my car regularly. The reception is reliable here all over Southern California and I can listen to whatever I want when I please.