That made me curious so I removed Flash from /Library/Internet\ Plug-Ins/ and rebooted. I'm unable to play video on either Vimeo or YouTube so I'll be sticking with Click to Flash for the moment.
Vimeo: Right below the description, see: http://imgur.com/yuf4R
Obviously not an automatic fallback but I guess that's because it is still in 'beta'.
Youtube videos with ads won't work, or embedded ones (I think the same goes for embedded vimeo vids)