Button action for the home button (in ICS), is automatically set to bring you back to the "home" panel. I don't know that you can even change the action of the button to "previews" with the stock launcher. I do know you can do that with third-party launchers, so are you using a third-party launcher?
Nova...that'll do it. It's in the Nova Settings menu under Gestures and Buttons, Home Button (you have it set to "Show Previews". Then below that setting is the "Only on Default Screen", "press home button once for default screen, then again for action", and you probably have that box un-checked.