IBM has an article for developers about developing cross platform applications useing .NET. Well, using Mono.

As someone who has (poorly) written a cross platform application using Mono I can say that it is definitely easy. As long as you keep portability in mind when programming the cross platform bit should fall into place without any additional work.

The problem that I ran into was the fact that my application stopped working correctly on newer versions of .NET and now only it only runs well on Mono (on Windows and Linux).