Just saw this in my inbox:
Buy.com - Logitech Wireless Bluetooth Music System for PC - 980414-0403
Your PC/mp3/stereo outputs to the Bluetooth transmitter, and the receiver goes into the aux line-in on most boomboxes. This is the probably the most flexible solution, as you can use any existing boombox. If Logitech's system anything like their headphones, the sound quality is excellent.