The hot corners just start the screensaver, or some other nifty functions you can choose, at least on 10.5.8. The "All Windows" option, ExposÃ© really, is great on the Mac Mini.
Snow Leopard on my iMac lets me put the display to sleep using a hot corner.
EDIT: I see now that you can choose the hot corners in ExposÃ© in System Preferences too, and one of them is display sleep.
Sleep (as in this thread) in the Apple Menu puts the computer to sleep, like closing the lid or tapping the power button and choosing that option does.