Same here...
Same problem here.
2011 iMac 21.5", BTO 2.8GHz i7, 12GB RAM, 1TB HD, Lion 10.7 (upgrade from SL).
From a fresh power cycle, everything behaves as it should. The problem will only occur once the machine has been woken from sleep mode.
Playing any video content once the machine wakes from sleep will cause a complete freeze of all UI elements apart from the mouse pointer. I can move the pointer, but cannot interact with anything (menus, Dock, nothing). The iMac will also not respond to any keyboard input. Holding down power for 5 seconds is the only solution I've found.
Music will continue to play in the background, as will the audio track to whichever video file caused the UI to freeze.
I do not believe this is a Flash issue. My iMac behaves like this for any video content (played in iTunes, from Finder, etc), occurs if I attempt to play HTML5 video content in place of Flash video in the browser, occurs irrespective of the version of Flash installed (including the beta 11 64-bit release) and even occurs if I completely uninstall Flash.
Never experienced anything like this in Snow Leopard so it's definitely new for Lion.
I would guess this is a video driver issue specific to the latest iMacs related to sleep mode.
-Dan