However, getting the GUI components to work (there's some instructions in the Mono developers notebook on this) is not simple OR platform independant as such since you seem to have to pick at source level whether you are using Windows.Forms or an alternative....

C# on OS X is just not there totally yet, but I'm sure the smart people working on Mono are well on their way to making that an erroneous statement.

However I'd also argue that the core lessons you are trying to get across to CS students would be of the 'Object Orientation' form, rather than focussing on one language/OS over another. I'd take a developer who knows OO and patterns and has a broad experience of their application in different languages any day over someone who was singularly in the Java or C# trench.

The 'why' is usually a much harder lesson than the 'how' - take it from someone who can easily write an article on how but has to put a ton of effort in to explain a why :)