I had a simliar problem once before, and when I went into the Preferences to check my scroll settings, not all of my Mighty Mouse functions were there. So I unplugged the mouse, and restarted my MBP, and once it was back up, I plugged the mouse back in and everything worked like before (didn't even have to mess with the settings). If that doesn't work, try unplugging the mouse, shutting down the computer, plugging the mouse back in, and booting up from there.