We'd downloaded a couple of rental films to watch. Started watching the first. "This computer is not authorised to play this movie". Um, yes it is. Really. "Do you want to open iTunes to authorise it?" Uh, we're in iTunes, but okay, go on then. "(opens iTunes) This computer is already authorised." See? See?
Try again. Same thing happened. And we just get gray instead of actual video content. Shut down and restart iTunes, and now it actually lets us play - from about a minute in. Click the "back to start" button, and it flips to the other film (thus activating the 48hr countdown to expiry).