Any mouse dragging is a problem in my experience, as sometimes people drag things without realising it. GNOME is as bad as Windows in this respect (panel is easily dragged to the side of the screen where it's very hard to drag it back if there are lots of icons - no spare space withouth icons! You can lock down the GNOME panel at least - see http://library.gnome.org/admin/deployment-guide/ for this and other lockdowns.
I've also had to write a simpler version of the standard GNOME clock applet which doesn't show a calendar at all, just the date/time - this was confusing someone who would click this without realising and then not know how to hide the calendar (it would help if it was a standard window with an 'X' to close). Fortunately PyGTK makes this quite easy.