I don't think this is going to help you write OS X apps in C#. It sounds like they are only open-sourcing the server-side code. This will allow you to run a .NET-based application server on OS X, but it won't let you build a WinForms or WPF app in OS X, for example.
However, the availability of a fully-supported .NET runtime on OS X could certainly make adding such support in the future easier.
You guys are missing some of the guts of the announcement. Microsoft is partnering Xamarin the company behind the Mono project. They have been providing .NET on non Windows platforms like Linux and Mac Os X for 14 years! For the last 5 or so commercially with products for Mac, Android and iOS.
They are using the just released Open Source .NET source code to legally expand their offerings and fix bugs and implement missing features.
Visual Studio will become Xamarin aware as standard and their products will be made to be easily added to the IDE.
By the way Xamarin do support WinForms on Os X with a recompile, but most people choose to implement a native UI calling Cocoa from c# on Mac.
http://tirania.org/blog/