Vala looks interesting.
It would appear that with GTK libraries available on Windows and OSX, a port of a Vala app should be trivial. Valac compiles to C and then GCC takes over for the rest of it.
It seems like the most well-known Vala apps are managed in San Francisco by Yorba, however they're only for Linux. http://www.yorba.org/projects/
Any examples of cross-platform apps written in Vala?
What's wrong with Vala?