I've had this happen a couple of times - screen saver kicks in, locks the screen, and when I put my password in to unlock it the system just seems to hang.
Cursor turns to the busy, HDD light flickers occasionally, but the only thing I can do at that point is power down.
This is on a desktop, not a laptop and according to the bios all the power saving is turned off.
Has anyone else seen this? Is there a way to prevent it, or at least diagnose the cause?