I use a Microsoft Natural Keyboard with my MacBook Pro - make that two, one at home and one at work. The function keys stopped working last week on the one at work which is a pretty big deal if you use Exposé as much as I do. I did the whole routine and could not find the problem. As the keyboard at home worked just fine, I started to believe that there was something wrong with the actual keyboard.
Then I came across this post which finally explained to me that I have a key right next to the F12 key named “F Lock” which makes the function keys do other things than I want them to do. A key stroke later all is good again.
The funny thing is that I have no idea how it got into this mode as I cannot turn it off again.