Mountain Lion Switcher (Cmd+Tab) not showing all apps
Solution 1:
I had this same issue right after I upgraded. In fact, the open app indicator in the Dock wasn't working for open apps either. I just restarted the computer and all is good now. The application switcher(cmd-tab) shows all open apps and the open app indicator in the Dock is working too.
Solution 2:
To avoid restarting, you can just kill the Dock process and it will start up automatically, showing all open applications. Open the Terminal (Applications -> Utilities -> Terminal) and type:
killall -KILL Dock
The Dock will disappear and in a minute, reappear.