Comment 5 for bug 57099

This would seem to work on Gutsy for switching to Sawfish as your window manager for Gnome.

Quit any applications you don't want to have running in your session.
Open "System > Preferences > Sessions > Current Session tab"
Find "metacity" in the list of currently running programs, and change the style from "Restart" to "Normal"; click Apply.
Start up a terminal and type in the single command line
  killall metacity ; sawfish &
In the Sessions list of currently running programs, verify that sawfish is listed now and of type "Restart".
Exit terminal (unless you want it too added to your current session).
In the Sessions dialog, switch to the Session Options dialog and click "Save the current session".
Done. You can close the Sessions dialog now.