There is a very experimental GUI/CLI for dynamically switching the number of running X servers. It uses the automatic configuration of the display managers (mentioned in Section 7.4), Python, dialog for the CLI, and Xdialog for the GUI.
Once it is more tested and bug-free, you could, for example, use it under Backstreet Ruby to switch between 2, 3 or more X servers and a single X server using Xinerama. So when your PC isn't used by more then one user, you could use the other monitors under Xinerama. Or one more funny example: you're simulating net gaming with a number of friends on your bruby Linux PC, you have invested a bit more in an additional graphic card which is already configured, but you don't have enough money right now to buy one more monitor and keyboard/mouse pair. One friend of yours comes and says, "Hey guys, that's cool. Can I join?" What would you answer? Using the GUI could result in the following answer from your side: "No problem, just bring your monitor,keyboard and mouse."
If you are feeling like a hacker and want to try out this BUGGY GUI/CLI, check the current status at http://karlovo.demon.co.uk/~svetlio/ruby-contrib/bruby-python/. But remember, it's not very tested, and if not configured properly it can cause you serious troubles. Please wait until it is more stable if you are not that familiar with Linux. If you feel comfortable enough under Linux, and think of yourself as a hacker, please help in testing it and making it better, bug-free and easy to configure.