The CB Bass group has been talking about a similar idea, only using the open-source Audacity package, which runs on Windows, Macs, and Linux (I'm a linux guy, and use Audacity as my main audio recording / editing tool).
The Kompoz site might be another alternative: there, rather than uploading the "project" file representing the whole song, you can upload individual tracks, which anyone can mix down in different ways. They have a pretty cool setup for synchronizing tracks, too.
I`m well game for that idea , I have a small home studio setup and mainly use Acoustica Mixcraft 6 (brilliant , easy and inexpensive) but have used Audacity before . Count me in , I play 3 string fretted , 3 string slide and 2 string slide (just switched from regular guitar so just getting up to speed)