Well I believe we're going to be enforcing a specific license (I think the choice was even CC-by-SA) for the new themes sight.
One of the intents of this is that if a theme becomes outdated, and the author is no longer around, we'll know absolutely that the theme can be updated. Of course another is that we like the idea of promoting the freedom to change, and basically "If we're providing space and bandwidth for the file, we'll decide what kinds of files we're willing to accept." So while we wouldn't be forcing theme authors to do anything, we'd simply be setting terms as to what can go on our hosting.