I was doing some tweaks to Ubuntu 12.10 and installed some programs, and when I logged into Gnome 3.6 the menu was gone. Nothing there, just the background. I can still ctrl-alt-T to get terminal or ctrl-alt-delete to log out. I have Cairo Dock installed, but I installed Gnome 3 shell after, and both worked fine. Then I did a few tweaks in Unity (enabled Hibernate in menu, removed tabbed scrollbars, and few other things). I deleted gnome screensaver and installed xscreensaver. I also installed Compiz, Unity Tweak Tool, Ubuntu Tweak Tool, and Gnome Tweak Tool. I think that is about all I can think of...anyway, Unity, Gnome Fallback, and Cairo Dock all work fine except for Gnome 3.
Found the problem. It was the removal of gnome-screensaver that caused the problem. I installed it back, and the menus and panel came back on. So, if you want to install xscreensaver, DO NOT remove gnome-screensaver as recommended by some people if you want to keep using Gnome 3.6. It does not affect Gnome Fallback.