It won't be as open as FOSS wants it to be to be 'open'. It won't be as closed as it was before. It might be good- but those two things won't make anyone satisfied, no matter how good 'good' is.
To me it's not so much that as I always worry about some little tricks others have tried before. Like release the libraries but not the APIs. Or open the libraries but not the language syntax or compilers, etc etc. etc.
Whatever Microsoft means by "open" I don't think anybody expects .NET to suddenly just be fully open (in the traditional sense) without some restriction or gotcha.
I personally think they're doing it to set the stage for claiming some sort of IP infringement somewhere. Because if source was made open, it only stands to reason (in Microsoft's mind) that anything remotely resembling a part they didn't release MUST have been stolen from them - or be infringing on a related patent.
Maybe they're getting ready to lower the boom on Mono (and by extension Linux) - as many of us have been expecting them to do.
Ballmer may be gone. But a company doesn't change it's mindset overnight.
So like I said, I'll reserve any judgment for now.
Beware of geeks bearing gifts.