You can make your Forms application and you develop and test it on Mono:
http://en.wikipedia.org/wiki/Mono_%28software%29[
^],
http://www.mono-project.com/[
^].
I would advise to start development on Windows and test it on Mono for Windows. You will need to keep your programming strictly in standards. Even though
System.Window.Forms
is not a standardized library, it is ported to Mono reasonably well. If you pass Mono for Window tests, most likely, it will work reasonably well on Linux, Max OS X, iOS (Apple platforms are most hostile to "foreigners" though), BSD and other platforms.
Good luck,
—SA