I've used this so many times today, I just have to share it.
Occasionally, when tabbing out of legacy games/SDL/dosbox/etc. under Vista (on my machine? New in Vista sp1 beta? With the beta nVidia drivers?), the screen will go completely black, but everything underneath will continue working, the mouse cursor will continue to move and change.
Ctrl+shift+esc (show task-manager), start+d (show desktop) and the usual set of keys don't make the black go away, nor does Ctrl+alt+delete.
The trick is to lock the machine (start+L), the screen will remain black, it's useful to have sound on at this point to ensure that it actually locked (ie. you aren't about to type your password into IRC :) ) and log back in again. Everything will return to normal. I have no idea why this fixes it. :)