.NET Framework 2.0, Mono, MonoDevelop, Mac OS X / Cocoa#
The current Mono HEAD compared to .NET FX 2.0 is still aways off, but progress is being made at a very quick pace. Our company just recently got some Mac mini's for development. I started looking into Mono progress on Mac OS X. Mono 1.1.3 runs very well on Mac OS X. The most wanted lacking feature being System.Windows.Forms right now. Cocoa# will allow you to use Cocoa in your .NET applications. This will of course tie you to the Mac OS X platform, but it is still a great option to have. This works just like the Java/Cocoa bridge.
MonoDevelop can also be used on Mac OS X. It requires an extensive install process though. One thing about the Mono project is the documentation is not accurate and not well written in some cases. You can find a lot of articles to help you on these subjects. I will be posting more info on my blog as I get accurate sources of information. I will also be making a MonoDevelop package for easy installation on Mac OS X.
Things are looking great for cross platform .NET development!
Here are some links for further investigation:
Mono Project Home Page - http://www.mono-project.com/
Managed Windows Forms Home Page - http://svn.myrealbox.com/blog/
Cocoa# Home Page - http://blog.sublimeintervention.com/
Mono SVN Sourcecode Repository - http://svn.myrealbox.com/
MonoDevelop Home Page - http://www.monodevelop.org
Have Fun!