Sporty Bluetooth
ModTown reviews a bluetooth "sports Headphone" from Bluetake. As it can be used as a headset for both your mobile and your MP3 player (it includes a bluetooth adaptor as well) apparently at the same time (up to 8 devices), this is the sort of device that I've been looking for for a couple of years now.
Now all I have to do is find someone that sells them…
