If you were going to develop a mono app on osx you'd be better off using monomac than gtk#. Mono develop is written in gtk# and cut and paste works just fine on osx.
-Build a GUI app in C# -Run in on both my Windows dev machine (Windows 7), and my main desktop (OS X)
That's it. I don't care even what I have to use to build the app (Windows Forms, Stetic, etc).
Any advice?