I've had a few occasions where most Mac OS X keyboard shortcuts suddenly stopped working (e.g., F9 for Exposé, F12 for Dashboard). Strangely, Copy, Paste, and Cut continued to work. The only solution seemed to be a reboot.
I discovered the cause yesterday: Dashboard. There must be some circumstance where it must not properly release the keyboard grab. Fortunately, I also discovered that explicitly invoking Dashboard (by double-clicking on Dashboard in Applications) cures the problem.
[robg adds: I haven't experienced this problem myself, but I'd be curious to know if others have, and if this fix works.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090409105032343