This knowledge base article keeps me up-to-date:
The SQL Server 2008 builds that were released after SQL Server 2008 was released
http://support.microsoft.com/kb/956909/[/url]
And, this one is useful, too, for both SQL Server 2005 & 2008:
Where you can find more information about the SQL Server 2008 builds that were released after SQL Server 2008 and the SQL Server 2005 builds that were released after SQL Server 2005 Service Pack 2