Get this before Apple pulls it from the App Store: Live Lock Screen by developer mix1009 will automatically change your lock screen wallpaper every 5 seconds (by default, you can change the speed in the app) on you unjailbroken iDevice on iOS5 only.
It does this by grabbing pictures from your Photo library and rotating them on your lock screen. You can customize the update time from one second to an hour, display the photos in random or sequential order and time it to stop after a certain amount of time. One downside to this app, is obviously its going to kill your battery if left on for a long time, but still is pretty cool.
You can get Live Lock Screen from the App Store now for $.99