Further to my earlier posts, I've been improving the documentation for the Inhibit and UnInhibit methods, and really testing them hard. There's now a testing utility, gnome-power-inhibit-test in g-p-m CVS that allows you to play with multiple Inhibits and other issues. The testing utility shows how easy it is to add the DBUS methods into your program. The same methods can be used with GNOME Screensaver (but you'll have to change the service and interface names to ScreenSaver). Trying to suspend when we have an multiple Inhibit requests gives us this. 
And a single inhibit: 