Sorry to be a little late on this bandwagon... was away for a while.

There was a collaboration just like this on vdrum.com. There are a couple of threads about it and how it was made.... if you don't mind going through 100's of post pages!!

Basically, what they did is record a bass line with a click track. That bass line is available to all. Anybody can just record and extra track... doesn't need to be the whole song, it could only be a riff, or the chorus, etc...

The click track is really needed because if people just submit wav or mp3 files at different tempo, it is impossible to join them and make any sense about them.

All tracks are then submitted to the master editor who selects the tracks and put it all up in one video or mp3 for youtube or anywhere else.

Check out the second vdrums collaboration here:

http://www.youtube.com/watch?v=1IGVq8VBvSI

There is also a band called the iGenesis, musicians from around the world playing Genesis songs, and they are amazing:

http://www.youtube.com/watch?v=1QaE-w73iBo

For sharing mp3s, I suggest soundcloud.com. Very easy to use and great for musicians to share their mp3s.

My new venture: http://www.voglosounds.com