Michal Zalewski has problems with Microsoft. He thinks Microsoft doesnt cooperate with the security research community, that in fact it abuses that community, that it disserves its customers and doesnt take security seriously.
For all this, when he discovered a bug in Internet Explorer that caused it to crash, he decided not to disclose it to Microsoft through the normal channels (email@example.com) but instead to disclose it publicly on the popular Full-Disclosure mailing list.
Its important to note at this point that, for all the use of terms like "highly critical," theres still no actual claims by anyone that they have been able to exploit the crash in order to execute attack code. Zalewski is very clear about this himself, and much more reasonable in his evaluation of the severity of the vulnerability than some outside agencies like Secunia.
Many crashes like this turn out to be exploitable, but not all of them do, and some are only exploitable with great difficulty. In other words, this may turn out to be nothing more than a way for a Web page to crash your browser.
Or it might turn out to be exploitable, in which case its a way for a Web page to execute arbitrary code on your browser, including installing programs.
Lets assume what is likely, that Microsoft found out about this vulnerability the same time as everyone else, when the company read it in Full-Disclosure. That means that Microsoft has only had since then to research it and work on a patch.