In the perfect world that exists only inside my head, all cell phones and MP3 players incorporate A2DP, also known as stereo Bluetooth. Consequently, they're all compatible with stereo Bluetooth headsets like the Jabra BT8010, which CNET rated 8/10, and which CompUSA and TigerDirect (different outfits, same owner) currently have on sale for $37.99. That's a pretty big savings over the original $149 list price.
During business hours, the BT8010 functions as a fairly standard headset, albeit one with a cool OLED display and jog dial. When you want to get your groove on, you just connect the second earpiece and presto: stereo earbuds. Once you've experienced the joy of walking around with your phone in your pocket and music piping wirelessly into your head, you'll never go back to corded headphones.
Just make sure your phone supports A2DP. Many models do, but for some reason it's not available on all handsets (cough iPhone cough). And it's pretty much AWOL on MP3 players, which mystifies me because, as I may have mentioned, wireless earbuds are suh-weet.
I've got a BT8010, and I'd rate it 7 out of 10. The headset is a overly susceptible to ambient noise and a little uncomfortable over long stretches. But for $37.99 (plus a reasonable $6.99 for shipping), it's a hard deal to beat.