My thought was that if Osama were alive, he'd have been making videotapes, just like his top lieutenant. My guess now is that someone just misplaced the camera.
Al Zawahiri has really not made that many videos himself since Sept 11th (two, maybe?).
Videos are meaningless in determining if one is alive or dead. The fact is, intelligence sources (or captured Al Qeade) would have given up the fact long ago if UBL was dead. Secrets like that just don't stay dead for long. (especially when pressure is being placed! - and I don't need to say "how").
UBL has been known to be alive since early 2003. I feel anyone who is a serious follower of "reading between the lines" could very well have seen this since early 2003.
Nonetheless, UBL is on continual borrowed time. It is just a matter of our shooters zigging when he zags the wrong way ONE time - Our boys will have him for lunch then!