This is huge!
Apparently Microsoft has seen which way the wind blows and had a genuine "come to Jesus" moment. In addition to the European Union's efforts to push MS into true interoperability, something fundamental has changed within the company.
I suppose this could be more smoke and mirrors than reality, but I don't think so. They seem really, really serious about it all. The company's press release/announcement is here. and there doesn't seem to be any funny business going on at all. I believe them.
Their covenant not to sue seems all on the up and up and offers rather generous terms to open source companies. Individuals and small organizations fly free.
Lastly; There is a certain smugness at Red Hat, Canonical/Ubuntu and some other Linux organizations. Their refusal to sign patent pledges with Microsoft is now completely vindicated. A short commentary on that can be found here.