I had this problem in Snow Leopard and hoped that it would disappear when I upgraded to Lion, but no such luck. Every time I attempt to watch an iTunes movie rental I am told that my computer is not authorized to play the movie. I attempt to reauthorize and it tells me that your computer is already authorized and that I have 4 out of 5 computer authorized on this account. I try again to play the movie, and just a grey window. The only way I can ever get movie rentals to play is to temporarily stop the movie download, restart the computer, and open iTunes, and then the movie finally plays. I have to do the same thing every time I rent a movie. It would be really nice to not have to restart every time I want to watch a movie. Any suggestions?