I see this as great news.

It could possibly result in a fast, (hopefully with Mono)
cross-platform Ruby VM.

More importantly, it would hopefully give Ruby native threads, and
robust database drivers. Two things it's really hurting for IMO.

There are lots of potential downsides, like the whole "embrace &
extend", especially since .NET development is so _incredibly_ complex
for complexity's sake right now. It'd be a real shame to see that
mindset penetrate Ruby. But no need to be negative. Ruby is Ruby, and
unless MS plays nice, any Ruby.NET "official" implementation won't be
anymore the thing that MS forces on the Ruby community than IronPyton
is the thing that MS forces on the Python community. That is to say, if
you like Python, and work with .NET, you probably appreciate that
there's an IronPython, but IronPython isn't the "official" Python, and
it's not what all the books and community are about.