In tmux 1.8 and earlier, periods were valid in session names. I found this useful, as I use sessions that map directly to directories on the filesystem, and I have projects named things like 'xiongchiamiov.github.com'.
As of tmux 1.9a, attempting to create a new session gives an error:
[$]> tmux new -s a.s
bad session name: a.s
I don't know if this was intentional, but I didn't see it (or anything seemingly related) in the changelog.