In the perfect utopia that exists only inside my head, all cell phones and MP3 players incorporate A2DP, also known as stereo Bluetooth. As a result, they're all compatible with stereo Bluetooth headsets like the Plantronics Voyager 855, which CNET rated 8/10, and which Newegg currently has on sale for $57.99, shipped. That's a pretty big savings over the $149.95 list price.
During business hours, the Voyager 855 functions as a fairly standard headset, albeit one with a cool sliding boom mic. When you want to get your groove on, you just connect the second earpiece and presto: stereo earbuds. Once you've experienced the ecstasy 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 Palm Centro cough). And it's pretty much AWOL on MP3 players, which mystifies me because (as I may have mentioned) wireless earbuds rawk.
Anyway, I've used the Voyager 855 myself, and while I like the noise-canceling nature of the earbuds, I agree with CNET's reviewer that sliding the boom mic does kill the fit. Still, music sounds great, and $57.99 is less than you'd pay for many non-stereo headsets. And now, back to my personal utopia, where my 60GB iPod touch supports A2DP. Ahhh.