The Yamaha BodiBeat MP3 player won't be out until late this year, but it's got a certain something. (And I don't just say that because of the running girl and bouncy music on their Flash site.) The BodiBeat analyzes your music collection, then syncs up songs that match your running tempo using a built-in accelerometer. It's also got a heart rate monitor that will clip on the ear if you're doing something low-impact.
It's a solid idea, but the implementation will make the difference. Yamaha talks about changing tracks mid-song if your pace changes, which could get irritating pretty quick if not handled with some intelligence.