Courtesy of Ben Lam’s question and Arvind Shyamsundar’s answer comes the following list of resources for keeping up with SQL Server product update releases:

• You can sign up for security bulletin-related email alerts from Microsoft here (Windows Live ID and Windows Live Alerts account required).  Be aware, though, that there are no product-related filters on this service.  If you sign up, you’ll receive notification of every security bulletin.
• The SQL Server Release Services team blogs here.  While this is a quasi-official channel at best, of late the team has been very diligent about posting release notifications.
• A third party site (free; registration required) crawls the Microsoft Knowledge Base daily and sends email notifications of new articles.  This service offers an extensive list of product-level filters. 

Thanks to Arvind for sharing this information within the Microsoft community, and for consenting to my disseminating it further.

-wp