Many users are reporting a similar problem after upgrading to Lion, where their late model iMacs will lock up after playing video. When this occurs the mouse pointer will still move, but nothing responses, and a forced reboot is required. After rebooting, there doesn’t seem to be anything pertinent written to a log indicating what happened.
This problem has been reported with playing different kinds of video, including Flash, YouTube, and media purchased in iTunes. Rumor has it this is one of the fixes the forthcoming Mac OS X 10.7.1 update will address.