Just spent a couple of hours on a real head-scratcher: for no reason I could easily fathom, all my modifier key shortcuts (command-space for magnifying glass, shift-click to select multiple objects, etc.) stopped working in FreeHand and Photoshop yesterday. Only then they'd work again. And then they wouldn't. I hadn't installed any wacky bits of software or changed any Universal Access or Keyboard shortcut prefs - they just wouldn't work, except in the Finder. I rebooted, fixed permissions, updated prebinding, plugged in a different keyboard, switched USB ports but, nothing.
Then I realised - the keys only stopped working when Terminal was open and, lo and behold, I'd switched on 'Secure Keyboard Entry' in Terminal's File menu after reading that it was fixed after... you guessed it, installing the 2004-12-02 security update. So, if you're having the same problem as me, switch off secure keyboard entry and all will be well.
This cautionary tale brought to you by Hans Christian Andersen Consulting.