Brandon Watson, a senior director in the Windows Phone group, said in a blog post Wednesday that that Microsoft started pushing out on Tuesday. "The rubber meets the road today," he wrote.

In early April, developer Chris Walsh that people could use to force updates to their Windows Phone 7 devices. He built the tool following delays and problems that Microsoft itself encountered in trying to deliver the updates, including an update that added the ability to cut and paste.

At the time, Microsoft warned users that the unauthorized update mechanism might create problems. With Tuesday's security update, which fixed nine fraudulent third-party digital certificates, some of those users are now indeed facing problems.

Watson explained why the security update won't work. The update process first checks that the phone has an official software image. Phones updated using Walsh's tool don't have an official software image so they won't be able to download the update, he wrote.

"Due to scheduling of engineering resources, we did not anticipate having to [undo] the changes made to phones by these unsupported methods. While we are not ruling out having a fix in the future, for now there is no fix," he wrote. "Unfortunately, our engineering priorities are focused on improving the process by which updates get to Windows Phone, issuing the security update you just got and working to getting Mango to market. Undoing this specific problem was not in our schedule."