GUI Quick Swapping

There is a need for graphic user interface swapping. One that will allow users to switch GUI's either by:

-switching user accounts:
For example: I am running Ubuntu 7.10...I want to switch to KDE GUI and not have to go through a long process to swap the GUI...I can switch my user account setup to run with GNOME the way i want...and then switch to my account with KDE.

Alternatively, provision for GUI selection at login could be very helpful...and may allow for a single user account to switch by simply logging out and selecting the desired GUI and logging back in.

-A session style manager or "theme selector" type manager. (simply open the panel and select the GUI of choice...Ubuntu goes into a, "Please wait screen"...and the screen fades back into reveal the desired GUI)

ALSO:
What are some thoughts on running multiple GUI's simultaneously on separate desktops...with Compiz Fusion or a similar program?

 I am aware that while running a GUI other parts of GUI's are running in the background or at least parts are often implemented for programs designed for that particular GUI to run.

But I am referring to actual desktops that are different
(ie: desktop 1=GNOME...desktop 2=KDE...desktop 3=Xfce)
Could this be done without running simultaneous copies of Linux or using virtualization???

These are ideas...water them...grow them...run with them.