This may seem like a corner case, but it's a bit frustrating and seems like it might not be too hard to fix.
I work in a mercurial repo in my Dropbox so I can go from home (on a Mac) to work (on Windows 7) and have my source files automatically sync.
Unfortunately, every time I run tox on one platform, it completely blows away all the files from the other platform and rebuilds everything. This is slow and makes it impossible to do certain convenient things, like set up tools to use the tox virtualenvs (since they keep being deleted!).
It would be nice if tox set up its files/directories so virtualenvs from different platforms could co-exist in the same .tox directory.