I upgraded four Macs to 10.8.2 (iMac, 2 MacBook Pros and Mac mini) and the Mac mini is the only one where the screen saver stopped working. Too bad, because this one is our media center, hooked up to a projector and a small 12" display. The 12" display stays on, displaying the photo montage screen saver (Vintage Photos), and works really nicely as a photo frame.
I've tried a bunch of things, including repairing permissions, restarting, changing screen savers, and it started working once, but then it stops working again.
Seems like theres a bug in the timer that tracks mouse inactivity or something, as the screen saver fails to activate.
Okay, finally Apple has released 10.8.3. Updating as I type, fingers crossed this fixes this for good. Will report back.