Movie clips make great fodder for animated userpics. The problem is the trailers are gigantic. So you have to learn to be very selective in the frames you grab. Here's a clip from Escaflowne that turned out perfect for a loop. Other clips look jerky when they reset to the beginning.
Two programs support the importing of Quicktime and other video files: Imageready and Ulead GIF Animator. Jasc Animation Shop will import MPEG and FLI/FLC files, but not the more common Quicktimes movie trailers. When you've gotten the video clips imported, you'll want to resize them and remove a lot of extra frames. A good rule of thumb for movie clips that you should use is to expect about 10 or 15 frames as a 100x100 GIF animation.