If you want to assign X11 to a Space, go to the Spaces System Preferences panel, and choose to add an application (click the plus sign). Instead of choosing X11.app from the Utilities folder, press Shift-Command-G to bring up the Go to Folder text entry box. Type /usr, and click Go, then choose X11 » X11.app. Assign it to the Space you wish, and you're done.