Microsoft today unexpectedly delivered an update for the Beta version of Internet Explorer 9, the first update since the initial release in September. According to the company, this update "improves the reliability of Internet Explorer 9 Beta in various scenarios," which isn't too much to go on. If IE 9 Beta is already installed, you'll see it in Windows Update.
Here's a bit more info from the IE blog:
Today we released a recommended update for all Internet Explorer 9 Beta customers via Windows Update (KB2448827). This update includes stability fixes for the beta build. It is not a new Internet Explorer 9 Beta build. Another update shipped today (KB2452648) resolves issues between the IE9 send feedback feature on 64-bit machines with Windows-Live-Essentials 11. These updates apply only to Internet Explorer 9 Beta.
Customers who have automatic updating enabled will not need to take any action because this update will be downloaded and installed automatically. Customers who have not enabled automatic updating need to check for updates and install this update manually.