I haven't seen this occur with my 10.7 installs. Does it happen even if Flash content is not displayed?
No it doesn't happen unless there's Flash content. It seems to happen most when I have a browser windows open on both monitors. Especially when I make a flash video (live.twit.tv for example) full screen on one monitor.
That's very odd. I use two or three monitors all of the time and haven't seen this with my Macbook Pro. Could you please open a new bug report on this over at bugbase.adobe.com? Please post back with the URL so that others affected can add their comments and votes.