(I asked this question in another way, and got some interesting responses but I\'m not too convinced.)
Is Mono\'s GtkSharp truly cross-platform? It seems to be Gnome
The best example of a Gtk# app that runs on both Windows and Linux may be Medsphere's OpenVista. Granted, its not an app that many people need to run, but it is a very professional, polished, open-source Gtk# application. It shows how a professional Gtk# app can be written.
http://medsphere.org/community/project/openvista-cis