I started contributing to the Mono project recently. Nothing big yet, but it is a start. I contributed an updated System.Diagnostics.DebuggableAttribute class with 2.0 features. It is fun working on a project like Mono and slowly seeing a majority of the code starting to work with current .NET Framework binaries. They have mcs, gmcs, and bmcs compilers now. The mcs being the C# compiler, gmcs is the C# with generics compiler, and bmcs is the inital Visual Basic compiler. Mono also has moved toward a much better System.Windows.Forms implementation. They are making backend drivers for each platform UI. They have a native one for Windows, X11, and OS X. This will allow your app to run seamless and have the native look and feel of the UI platform it is being executed upon. A lot of this is possible because of a gentlemen by the name of Geoff Norton (http://blog.sublimeintervention.com
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