QUOTE:
"In the last few days, we have uncovered a compatibility issue between Microsoft Dynamics Retail Management System (RMS) and both Windows XP SP3 and Windows Vista Service Pack 1 (SP1). In order to make sure customers have the best possible experience, we have decided to delay releasing Windows XP Service Pack 3." -Chris Keroack, XP SP3's release manager

Microsoft is going to place a filter so that machines running RMS can't see the update, and only then release it to Windows Update.

More information at Computerworld.