I'm on Ubuntu 12.04 and using GNOME 3. This morning, after an update and a reboot, there was no longer a panel on the desktop. I have two monitors, if that makes any difference. I tried uninstalling any gnome-shell-extensions (I have gnome-shell-extension-prefs and gnome-shell-extension-tool) but it couldn't find any package by those names. I'm not even sure this is the reason that the panel is gone, it happened after an update.