Why are my top-row keyboard buttons not working on Mac Lion?
If you check the Keyboard pane in System Preferences, you should see an option called Use all F1, F2, etc. keys as standard function keys. I think you'll find it checked. The default is to have it unchecked.

Then within the Mission Control pane in System Preferences, I think you'll find that "Show Dashboard" is set to F13. The default is F12 (note that the default doesn't work if Use all F1, F2, etc. keys as standard function keys isn't checked).