The Microsoft Download Center page, Toolkit to Temporarily Block Delivery of Windows Server 2003 Service Pack 1, contains the following description:

"While recognizing the security benefits of Windows Server 2003 Service Pack 1 (SP1), some organizations have requested the ability to temporarily disable the automatic delivery of this update through Automatic Updates (AU) and Windows Update (WU)."

You can download the toolkit from http://www.microsoft.com/downloads/details.aspx?familyid=FC145B0B-C148-445A-82BA-9B2F3AEF6E60&displaylang=en.

The mechanism that temporarily disables delivery of Windows Server 2003 SP1 will be available until March 30, 2006.