For Macs configured to activate Exposé with a desktop hot corner, it is possible to leave the "exploded" Exposé view open indefinitely. It turns out that this behavior overrides both screen saver and display sleep settings. I realized this today when I got up from my Mac, absent-mindedly left the mouse cursor in the corner -- activating Exposé -- and then leaving to run some errands. When I came back an hour later the screen was exactly as I left it, but the screen saver should have come on within five minutes, and the display should have been put to sleep within ten.
I've duplicated this behavior with both "all windows" and "all application windows," and got the same result either way. Whether or not this qualifies as a bug is a matter of opinion, but it was certainly not the behavior I was expecting. Those that count on things like screen saver passwords for security and a powered-down screen for battery life or general power conservation should be aware of this side effect of Exposé activation.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031107104324301