(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
It would be more correct to say that GNOME is GTK-based than it is to say that GTK is GNOME based. GTK is a toolkit that GNOME sits on top of, and you can get GTK for several platforms, including Windows. That's how GIMP works on Windows: you install GTK first.
Gtk# is cross platform. However the only platform where it looks nice is Linux/BSD running GNOME. If possible somehow, separate frontend and backend and develop separate user interfaces for Linux, Windows and OS X. Even wx, which does a really good job in looking okay on all three platforms, has its limits.
Working Mac/PC/Linux app in Gtk#? Tomboy runs on all three I think.
Plastic SCM is supported on Windows, Linux, Solaris, and Mac OS X. The link includes screenshots on Windows and Linux.
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
Realize this is now an old question, but Banshee fits the bill for being a cross-platform application that uses GTK#. It runs on Max, Linux and Windows. http://banshee.fm/download/