According to the Wikipedia page on DotGNU [ http://en.wikipedia.org/wiki/DotGNU ], Mono doesn't just aim to by compliant with the Common Language Specification, but to maintain compatibility with .NET releases as they come out. DotGNU, on the other hand, is only concerned with implementing the standard. So, at least in theory, Mono could get into trouble where DotGNU wouldn't.

Still, this isn't making a lot of sense. My understanding is that Microsoft has been very helpful to the Mono project. But what really gets me is "it's a really good thing to have free implementations of C# but a really bad thing to use them..."