First it was the Silverlight update rollup, next it's the Forefront 'Stirling' beta.  Over on the WSUS product team blog they state:

"In the next couple of days, we are adding a new product family to your WSUS server -Forefront Code named Stirling -Beta.  The Forefront Code named Stirling -Beta product family will include updates for all Forefront Code named Stirling -Beta products, including Service Packs, Optional updates, Definition updates and Critical or Security updates. For additional information on Forefront Code named Stirling -Beta Products see: http://www.microsoft.com/forefront/stirling. These updates will be available through the Microsoft Update Site, and making it available (depending on your settings) via Automatic Update, and to Windows Server Update Services (WSUS)."

To continue reading click here.

Enjoy!

J.C. Hornbeck | Manageability Knowledge Engineer