If you can't figure anything else, move the Gnome-related (or all, if you're in a hurry and don't care just now) "hidden" directories (directories whose name begins with a dot) from within your home directory to a temporary location. This is effectively the same as erasing Gnome-related (or all, if you moved all) user configuration, i.e. reverting your desktop to the default settings. After this you should be able to login. You could just remove the files, but moving is better because if you notice you just lost something important, you can move it back (which you cannot do if you remove the files). If you wonder how to do this without being able to log in and start anything, just (at the login screen, for example) press CTRL+ALT+F1 and you should get to a text login console -- log in and start working. To list the dotted files under your home directory,
where '-d' makes ls print just directory names, not their contents as well (and $HOME so it works even if your current working directory wasn't your home directory at the moment, as it usually is after logging in). See what you think you need to move, make a (temporary) directory and move. For example to move .gnome2 and .gnome2_private,
Code:
mkdir $HOME/MOVED_DOT_FILES
mv $HOME/.gnome2{,_private} $HOME/MOVED_DOT_FILES
Now MOVED_DOT_FILES contains the two directories (note how you can use {} to write the file list shorter in bash -- it extends to ".gnome2 .gnome2_private"). You might have to do this to some other directories, I'm not quite sure where all these desktop related settings are; a quick way is to move all ('$HOME/.*) dot-files, but then you'll either have to re-do all your custom settings like theme changes if you have them, or pick and move directories/files back.
When you think you are done,
then press CTRL+ALT+F7 to get back to X, log in as usual and see if it helped. If not, try moving other directories..
EDIT: you should probably log off from your Gnome desktop before doing this, because otherwise it might have some files in use when you start moving them around, and that might be a problem (or then not). A reinstallation of the system is like shooting a fly with a 155mm artillery, as this is clearly a settings-related problem which can be erased by erasing the configuration file, or in other words, to make Gnome forget it ever put the thing in use in the first place. I think it's a problem if something wrecks up the whole desktop just because you added something to panel, but this isn't a perfect world..and
if moving the files/directories does not help in any way, just move them back, overwriting any newly created files to get back to your initial configuration. Then think of something else
