Get a bluetooth adaptor for your PC, get a Pocket PC with bluetooth, use Bluetooth networking and play the files straight off the computer from the Pocket PC.
Failing that, I think there's something to do with Bluetooth Audio Gateway. I really havent played around with BT either much, but I know there's gotta be something like a set of Bluetooth headphones that will connect to an audio gateway which would be supplied by a USB Bluetooth adaptor. That is if you only use your PC for an audio source. This would be ideal around the house actually, but if your audio source needs to be ultra portable, I wouldnt have any idea. Besides, if your MP3 player is portable, why not get a case or some sort of belt clip for the MP3 player and run the cord underneath your shirt so it doesnt dangle and get caught on things?