I'm certain that this varies between distros, but with SUSE 10 it's in your /etc/sysconfig/windowmanager file. The pertinent lines are:
# Here you can set the default window manager (kde, fvwm, ...)
# changes here require at least a re-login
There's no "switchdesk" command in SUSE like there is in Fedora/RH, but off hand I can't tell you which file the switchdesk command in Fedora modifies. Poke around your system and remember that "grep" is your friend.