In Gnome 2.4 (currently in Debian unstable), you can right-click an individual applet (e.g. the menubar, task list, workspace switcher) and choose "Lock" to prevent that applet from being moved accidentally. In Gnome 2.6 (not yet released), you will be able to lock down an entire panel by setting certain gconf keys, and prevent non-root users from changing the panel.
Aha I have 1.4 on Debian so I guess I'll have to wait. Cheers though T