That would be sweet that feature is one of the differences I like most about the N8. The old Treo WInMo devices had it. It would be nice if this was added in WP8
There was another application that Nokia came out with for the N8 called Nokia Sleeping Screen that was pretty neat. You could load one of Nokia's preinstalled images or take an image from your gallery, pixelize it and use it. It worked real well and i didn't notice any excessive battery drain from it: