.NET isn't really the problem, the Xamarin guys have ported pretty much all .NET functionality we need already in Mono. The problem is winforms.

Winforms (System.Windows.Forms to give it its full name) is a .NET namespace which provides windows and controls for Windows operating systems. Buttons, toolbars, menus, statusbars, custom drawn controls, mouse and key event handling, tooltips, etc. etc. etc. are all winform classes in GH1.

On the Mac there are no Windows controls, instead you have to use Cocoa. So what we need is either a way to map all winforms code to cocoa code (no good 64-bit solutions exist), or a platform which can run on both interface platforms.

We're going for the latter option. Rhino6 for windows (and Rhino5 for Mac) will ship with a special build of Eto, whose aim is to provide most of the functionality in the intersection between Windows and MacOs. With Eto we hope to be able to develop .NET plugins with rich UI that run on both operating systems. It's too late to rip GH1 apart and translate all winform code to Eto code, but since GH2 is a ground up rewrite anyway there's no additional effort involved in switching platforms.

Sorry for the late bumb but just stumbled upon this thread.
@David: Sounds cool. Does that mean that Rhino + GH could theoretically be ported to linux as well?