The Photosynth Team describes a "hidden" switch that activates Autoplay on a Photosynth collection, triggering Photosynth to automatically advance through the images in a tour that will take you through every photo in the set.
Rob La Gesse, in the comments of the Photosynth blog, describes a VB6 app that he whipped up to turn Photosynth into a screen saver.
Rob, I covet your screen saver! Could you post it somewhere? :)
[Update: I wrote a PhotoSynth screensaver]