Could you explain a bit more in detail? I would really like to port it with the least effort.
Oh, there are so many. Basically, you switch your widget toolkit to something multiplatform and rely on those bindings instead of Microsoft APIs, ..., and then you're done. 
Here's a list of the most common ones to get you started on your research, since I don't know what your app currently looks like (source level):wxWidgets/WX.Net
GTK+/GTK#
Qt
FLTK
Also there are multiplatform frameworks available that target android and iOS. But i'll stick to the desktop version for now and make it platform independent... Challenging, but fun
As for Android/Apple... if there are any decent ones, I wouldn't rely on any of them at this point. For the most part, the answer is, "No." You have things like Appcelerator's Titanium to help get you up and running, but I would probably individually implement these as clients to web services and just keep them as light as possible. Mobile devices implicitly rely on connectivity, so I don't consider that a downside as many do.