Hi!
I've had the very same problem with my Lion install, but figured out a workaround.
If you feel comfortable with having the cam activate itself right after you enter the chatroom/start the webpage, then this will help.
In Settings in OSX go to Flash Player -> Camera and Mic ->Camera and Mic Settings by Site and on the right side of the site you want to use your cam with choose "Allow". It has to be a site which you already visited, otherwise there won't be an option to enable the Cam and Mic access.
This will permanently enable the cam whenever you visit the site eg. Chatroulette.
Also notice, that when you go to a site such as Megavideo and choose whichever movie, put it into fullscreen (given of course, that you use the flash, not the h.264 version of the video) and right click, you can manipulate the flash setting of this particular site however you wish.
On a different note - this problem has been noticed in previous versions of Flash, and isn't specific to Lion or Mac OS whatsoever - it has also happened in PCs running Windows. Just wonder how many users who upgraded to Lion on this forum have experienced this issue - is it widespread or just present among a given group of people.
Cheers!
Here's to hoping