For what it's worth, I'd been experiencing the same problem -- and only on my 2011 iMac, not on my other two MBPros.
I followed the advice of someone who posted much earlier, and in the energy settings have disabled allowing the computer to sleep (I still let the display sleep). Ever since changing that setting, no problems with the iMac freezing.
My guess is that this will be solved in some future Lion update, but until then I'm happy to no longer have to hard-reboot every time I watch videos. As I recall, it would happen when watching youtube and also sometimes watching embedded videos on websites. Annoying, maddening, but finally resolved with this workaround. For me, at least.