Apparently, after coming back from sleep state, Lion will freeze when attempting to run flash videos, where only the mouse cursor will move but nothing is clicakble. Cannot access Activity Monitor nor Force Close. The only solution is to hard reset with the power button.
I've experienced this myself multiple times, and so have others. If you're experiencing it too, please report at the provided links to Apple Support.
http://gdgt.com/question/anyone-else-having-flash-video-freeze-their-27-imac-with-lion-f4s/
Hopefully patch/fix is on the way...
EDIT: Report on TUAW: http://www.tuaw.com/2011/07/24/lion-bug-some-imacs-locking-up-after-playing-video/
I think I experienced this with a developer preview of adobe flash, but I think it was an upgrade. If it's an upgrade and you have this problem, then that's the reason. It doesn't do that anymore with me. Maybe even try re-installing the latest version. 3rd Party Plug-ins mostly haven't been released yet, still in beta. Unity Web Player 3.5 is coming out this week. Panda 3D still doesn't have it. Also, I have 10.7.2 on my computer and it fixes some of my problems too.