What about Mono? I have no idea how practical this is for real code bases, but I've used it successfully on several smaller projects.
One problem I'm running into at the moment is I'm trying to port a vendor's reference SOAP Service over to Mono/Linux and their provided secret sauce DLL is blowing up when calling their objects' constructors. I'm yet to pin it on Linux or Mono. Here's hoping the vendor is Mono friendly and willing to debug it. If not we're going to have one oddball Windows server in a sea of Ubuntu EC2 instances.
Ironically, we have some large-ish C# code-bases that run on everything but Windows.
The memory model is, IMHO, where Microsoft is pushing the limits of the existing C# language.
What has Microsoft changed in the .NET memory model that makes you think they're pushing the limits?
From what I can tell, the .NET memory model is one of the things that has changed the least. The languages have changed a lot faster than the memory model.