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. Because the OSP states that the promise is irrevocable, it may not be withdrawn by Microsoft. The OSP is, and will be, available to everyone now and in the future for the specifications to which it applies. As stated in the OSP, the only time Microsoft can withdraw its promise against a specific person or company for a specific Covered Specification is if that person or company brings (or voluntarily participates in) a patent infringement lawsuit against Microsoft regarding a Microsoft implementation of the same Covered Specification. This type of "suspension" clause is common industry practice.
IANAL, but I believe that what will happen is that those patent sales will be encumbered by the existing agreement, and accordingly less valuable.
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.
> 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. Because the OSP states that the promise is irrevocable, it may not be withdrawn by Microsoft. The OSP is, and will be, available to everyone now and in the future for the specifications to which it applies. As stated in the OSP, the only time Microsoft can withdraw its promise against a specific person or company for a specific Covered Specification is if that person or company brings (or voluntarily participates in) a patent infringement lawsuit against Microsoft regarding a Microsoft implementation of the same Covered Specification. This type of "suspension" clause is common industry practice.