When I log on to one of these machines with a "virgin" user account, select "Launch / Preferences / Desktop Preferences / Display / Themes", I get back a pop-up saying (roughly) "I can't do that because
either you don't have metacity installed or gconf is mis-configured".
SUNWgnome-wm is installed and metacity is running. All of the GNOME2 stuff is installed.
Certainly doesn't sound like any of the more common login issues I've heard of -- I'm not really familiar with jumpstart installs, but it certainly sounds like not everything has been installed that ought to have been.
Logging into a failsafe terminal and running 'gnome-session' from there may give more clues...