I had this problem only after installing Mono 1.9.1.

If you modify the ~/.bashrc and ~/.profile to remove all the references to the /home/YOURUSER/mono-1.9.1/* from PATH, PKG_CONFIG_PATH, MANPATH and LD_LIBRARY_PATH gnome loads just fine. Apparently anything but mono 1.2.6 causes it to puke.

Once Gnome is loaded and you need to set your path for some development, execute ~/mono-1.9.1/bin/setup.sh