Depends on your architecture and your definition of "easy". To put it another way, I imagine porting a MonoMac app to Windows would involve a similar level of

Depends on your architecture and your definition of "easy". To put it another way, I imagine porting a MonoMac app to Windows would involve a similar level of effort to porting a Windows Forms app to WPF, or vice versa. The more code that lives outside of the presentation layer itself, the easier. To my knowledge, code written for MonoMac will be very Mac-specific (you'll have lots of NS* objects in there), and so it's not a matter of flipping a compiler switch to get a GTK# app or something similar. You'll have to re-write those particular sections. Again, though, if you architect your project well, you can minimize the amount of re-writing that has to be done.