Realistically, Mac OS X is the only other platform Mickeysoft would considering porting to.
They could do it in a heartbeat; they just have to decide if it's of strategic benefit to do. For example, if .Net became a key component of either IE or Office, then it would happen.
As far as Mono is concerned, look at it as an open source implementation of C# and the CLR.
It won't run any proprietary .Net apps, and I figure most of M$ .Net development will "leverage" their proprietary libraries.
It seems a lot of people are confused as to the purpose of Mono. Mono isn't attempting to provide .Net for *nix operating systems. What it is trying to do is create a cutting-edge application development platform based on C# and CLR for *nix systems. There is a big difference.
Of course, I'm not yet a .Net expert, so these are simply my opinions based on what I've read.