It happened to me once too.
I switched to a terminal(Ctrl-Alt-F1 for example) and logged in as root to kill the screen locker app(I don't remember its name).
After playing with the power management and screen saver settings it never happened again.
Anyways, it could be just because I updated to KDE 4.0.2 (stable repository).
I guess what I'm trying to say is: update, play with the settings and remember that KDE 4 is not stable enough(yet) for day to day work.
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