I just received the following heads-up from Microsoft:
In the last few days, we have uncovered a compatibility issue between Microsoft Dynamics Retail Management System (RMS) and Windows XP SP3 and Windows Vista Service Pack 1 (SP1). Microsoft Dynamics RMS is a retail chain management solution for small and midsize customers.
In order to make sure customers have the best possible experience we have decided to delay releasing Windows XP SP3 to Windows Update and Microsoft Download Center.
To help protect our customers, we plan to put filtering in place shortly to prevent Windows Update from offering both service packs to systems running Microsoft Dynamics RMS. Once filtering is in place, we expect to release Windows XP SP3 to Windows Update and Download Center.
Until we have published a fix for this issue, we advise Microsoft Dynamics RMS customers to not install either service pack. Microsoft Dynamics RMS customers running Windows XP SP3 or Windows Vista SP1 should contact Microsoft Customer Support Services for additional information.
The fix is currently in testing and will be available as soon as that process is complete.
For further information regarding Windows XP SP3, please visit the TechNet Forum.
Note, however, that per my earlier news story today, Windows XP SP3 is still available for download via a direct link if you want it.