cmd+0 zoom not working

Command+0 is not changing the zoom level. Command+- and Command++ work normally. Command+0 does nothing.

If I go to the View menu in the browser and select "Actual Size" or "Reset", the zoom works normally and reverts to the default.

This occurs across Safari, Firefox, and Chrome, so I assume it's a system problem and not a browser problem.

I don't see any conflcting shortcuts in System Preferences-Keyboard. Macbook running OS x v10.7.4 Lion.

What's going on? and How do I fix it?