First I see this as a good thing. a) It's open source, how can that be bad?, b) Already I see some die hard Microsoft fanbois (yes, there are some of those left ) who swore they wouldn't touch JS with a 10 meters pole hailing this as the thing which saved JS. Good for you. Please use this.
I have said before on the IE blog, "Microsoft was brought into HTML5 screaming and kicking", after a long detour into Silverlight. So now I say: Microsoft came into open source screaming and kicking, but came it did. Welcome (not that I am a great OS contributor, or any contributor).
I'm just curious what's Anders, or Microsoft, take on releasing dotnet on the same terms, or when he came to realize that open source was any good. (I'm not saying that all the great guys who contribute to open source were like that for their entire professional life, but still, it seems Microsoft were last, please close the door after you )
EDIT: And for that matter, and i'm sure Andres isn't reading this, but what's your take on Microsoft calming Linux infringes on 287 or so of their patents??