Today while setting up and trying new application I found that one of the Gnome panels containing Applications, Places and System menu (taskbar for Windows users) wasn't displaying. After restarting the Gnome session I found an error message along the lines of: Error panel already running will exit panel. It repeated 6-7 times and then the none of the panels displayed! I tried several suggestions from the net and yet none worked. However I didn't believe nvidia drivers were the problem, as few suggested, and refused to reinstall them. And then I found the really simple solution.

On further exploration I realized Gnome sessions are saved in ~/gnome2/session file. I simply deleted the file and restarted the session. That's all it took to solve this problem.

Overall I think this simple step will solve many of your Gnome desktop related problems.