So you can build one .exe GUI app and run it on Windows, Linux, and even OS X. Craziness.
I've been thinking about the dozen ways to do this lately:
-- Qt 4.x which is now GPL on Windows. Pro: Mac, Windows, Linux, no dependencies. Con: C++, commercial for mac.
-- Perl-Gtk. downside: no mac, tons of dependencies, no good installers
-- Gtk# on Windows: runtime, installers
So it seems that System.Windows.Forms on both Mono 1.1.4 on Linux and the native Windows .NET libraries are the best bet. And C# as a language is pretty reasonable.
Alternatives? Think end-user experience.
P.S. If you install mono 1.1.4 and play with this, be sure to add this to $SYSCONFDIR/mono/config:
<dllmap dll="gdiplus.dll" target="libgdiplus.so.0" />
And make sure /usr/local/lib is in your library search path (LD_LIBRARY_PATH or /etc/ld.so.conf)