This came up in another thread, and I thought more people might be having the same issue.
If you're unable to fullscreen embedded youtube videos (Such as the ones on the forums, or in news posts), this is caused by the youtube HTML 5 functionality.
If you opt out of it at http://youtube.com/html5/ and click "Leave the HTML5 Trial", once you refresh the forums/newsposts/whatever has the embedded videos, you'll have the full screen button back.











