While not exactly what you are looking for, have you considered one of these?
http://www.thinkgeek.com/electronics/audio/64fb/
I use one of these with my iPAQ to listen to MP3s through my car stereo. It works good and will work with any radio receiver. The other nice benefit is that it will work in your house, at your friend's house, at parties, etc... If you really want to play your music anywhere there is a radio, this is the best choice (IMHO of course

).
Edited to correct my horrible abuse of the English language.