>Cross platform .NET usually means using a native toolkit on each different platform and the result is WPF on Windows, GTK# on Linux, and MonoMac on Mac.
As someone who worked on WPF though, I will comment that the code may not actually be too useful. It is heavily tied to Windows and its graphics infrastructure.
I'm more interested in seeing something like Moonlight ported to modern platforms.
Open Sourcing it is one thing, converting that into an engine that can do layout and rendering identically across multiple platforms is non-trivially difficult. Talk to the browser folks on how hard they had to work on that, and they wern't dealing with 3D stuff.