It's really been bugging me that, since updating to 10.6, my F8 through F12 keys sometimes won't work after a period of time. Recently someone told me how you can see when this happens.
Open Console, make sure the log list is visible, and then search for BUG in libdispatch using the search box. If you see this...
/System/Library/CoreServices/Dock.app/Contents/MacOS/Dock[1291]: BUG in libdispatch: 10C540 - 1676 - 0x4
...then you probably have non-functional F8 through F12 keys. To make them work again, open Terminal and type killall Dock and the keys will work again.
I didn't discover this myself, but though it worth sharing the solution.
[robg adds: I did a bit of digging with Google, and some other reports reflect that this may be restricted to those using the Chinese Input Method. I haven't seen it myself.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091203170258871