Killed by numlock
Powerbook users beware: it's very easy to accidentally hit the numlock key while the screen saver is active, and the numlock light is not very visible.
This causes part of the keyboard (the simulated numeric keypad) to appear dead when you try to type your password to unlock the screen saver, and you go what? dead keyboard?.
It happened to me in front of a customer lately, they were impressed with my sleek computer but the effect quickly faded away when I declared it had a problem I had never seen before ;-)
Ted Leung's story about only three characters showing in the password dialog sounds like the same thing: if some keys don't seem to work, check numlock!