This may not be exactly what your looking for, but xsm (x session manager) can sort of do something like that. You have to set up sessions, and AFAIK you can't change sessions midstream. Some window managers are "session aware" so changing your window manager will be saved for the next session -- provided you also don't have an entry to start a window manager in that particular session.
The problem with gnome and kde is they are finicky about their window managers. However, you can set up both gnome and kde sessions as well as bare bone fvwm. You chose your session after login. xsm is suggested by gdm in potato and the set-up works very well. I have flwm, Enlightenment and IceWm sessions managed by xsm, and then let gdm fire up my gnome session using sawmill. It's easy to use, check it out. -- +----------------------------------------------------+ | Eric G. Miller egm2@jps.net | | GnuPG public key: http://www.jps.net/egm2/gpg.asc | +----------------------------------------------------+