By default, Mac OS X has a keyboard shortcut, Command + H that hides any given application. In Photoshop, this shortcut has always been occupied to Hide Extras (View > Extras). Added in Photoshop CS5 however is the option of changing that behavior so that either OS X or Photoshop uses the shortcut. The loosing command will instead get the shortcut Command + Control + H.
When you hit the shortcut for the first time on CS5, Photoshop will ask you what to do and save that as a default. If you wish to change, it is available in the keyboard shortcuts.