I'm actually in the market for the same type of thing.. I use my mp3 player at work all day.. long story short, I have to wear it on my arm and run the cables up my sleeve to keep them out of the way, which is all fine and dandy during the summer when I'm wearing short sleeves, but come winter when i'm wearing long sleeves, I won't be able to see the screen to switch songs, etc.. something like that would mean I could have it transmitting from somewhere accessable and still have the cords tied up and out of the way.. not to mention I wouldn't mind not having to worry about bumping my mp3 player into things as I'm climbing in and out of airplanes ;)
been doing some looking and so far, all I've found is this:
http://www.engadget.com/2006/11/28/princeton-technologys-bluetooth-headphone-adapter/
more or less what I'm looking for, too bad it only works with ipods =(
if you find something that works with a stereo jack or stereo inputs of some sort, let me know will ya?
-Alan-