As an update to using C# on the Mac sometime down the road, it looks like Microsoft is already started. I didn't think it was any surprise that MS started with FreeBSD as the OS of choice for .NET after Windows -- It's completely "Free" as in "Free to take over as your own" (eg, Apple) and, well, obviously Apple's new OS X uses it.

At any rate, you've got ye olde CLI on OS X 10.2 now, and I'd assume Darwin as well. Under the current license there's not much you can do with this Frankenstein's monster, but it's interesting to see.