The Ctrl+Alt+F keys toggle you in and out of virtual terminals that are always running in the background. To "kill" and restart X you can press Ctrl+Alt+Backspace and it will restart X, returning to a login screen. To have multipe GUI sessions going at once you could use Xnest. There's an option in the Ubuntu menu already for it I believe you just have to go into the menu editor and check it so it shows up, it says something like "Open a new login in a nested window" or something like that. If it doesn't work you may need to run "sudo apt-get install xnest" or "sudo apt-get install gdmxnest" at a command line to grab the appropriate software. Anyway, after you get your "nested" window up, assuming you have the other desktop environments installed, you could just click "session" and select the other desktop environment and have that other session running in your nested window. Hope this helps.
Last edited by dudeman41465; 05-10-2008 at 10:10 PM.