Earlier quoted context omitted.
Allegedly it is legally binding! Scroll down to the bottom of the page, expand "OSP General" and you'll see a question "Is this OSP legally binding on Microsoft and will it be available in the future to me and to others?". The response is: > Yes, the OSP is legally binding upon Microsoft. The OSP is a unilateral promise from Microsoft and unilateral promises may be enforced against the party making such a promise. Be…
there is a catch, if Microsoft ever sell any of the patents to a third party, this is not binding to them.
If the new patent owner tries to sue people who relied on Microsoft's prior promises not to sue, it might be possible to accuse them (Microsoft) of promissory estoppel and force them to cover any damages. It's probably not in Microsoft's interest to play games like this.