it's an interestng idea.. however it lacks simplicity for everyone to make them...
they could be an option of a style of backgrounds. however it will probably suck a lot of CPU having a 'maybe' contantly rotating background that is handled frame-by-frame.
In fact I was thinking about something like a flash video player, I can see in youtube-like sites that Flash seems to be capable to move through videos without eating a lot of resources.
However, this kind of video tends to be extremely compressed and in order to have a decent image quality while the video is paused in any frame the video bitrate should be a bit high, maybe that could eat too much CPU, thought.