Write once, run on Mac & Windows? (C#)

I try to get into Mac OS X programing world recently. The link as below is the new oppertunity of C#.
Have a look!

English Version: Creating Mac OS X applications with MS Visual Studio Express http://mac.sillydog.org/dev/visual_studio.php

Chinese Version: http://www.iim.nctu.edu.tw/~toki/CSharpOnOSX/c%23osx.html

Maybe in the near futhure, we will see the the possibilty of "Write once, run on Mac & WIndows".
Of course, you can say Java already done this, but C# ? This is my point.