I have a MBP 6.2 running mac os x 10.6.8 and I've just recently starting seeing this problem.
When I go away from my Mac, I always lock the screen. When I come back and put in my password, the mac will randomly decide that my password is incorrect. This usually happens in the afternoon with the machine having been on all day.
There seems to be no pattern to this, but once it happens, I have no choice but to power the mac off and start again. The mac accepts my password for login. It will also accept it if I lock the screen, several time.
Any help with this? I'm afraid of losing data. So far, I've been in remote desktop sessions to PCs and on web-based apps and no local apps.