Silverlight works on OSX and Silverlight uses .NET Framework (at least a large portion of it).
Does it mean Microsoft actually ported .NET Framework to OSX but doesn't tell us? Or do you think we can accept an official .NET Framework for OSX any time soon?
A large proportion of .NET is in portable IL anyway. The important point is that they've ported the CLR - and we knew they'd done that anyway for the Shared Source CLI (aka Rotor).
I wouldn't expect an official release of .NET on the Mac any time soon though - I don't have any inside knowledge to the contrary, and I'd love to see it, but I don't think it likely.
There's a very good article on MSDN about how they refactored the CLR and the .NET framework for Silverlight.