Here's the official word, via a post to the TechNet XP Forum:
"Today we are happy to announce that Windows XP Service Pack 3 (SP3) has released to manufacturing (RTM). Windows XP SP3 bits are now working their way through our manufacturing channels to be available to OEM and Enterprise customers.
We are also in the final stages of preparing for release to the web (i.e. you!) on April 29th, via Windows Update and the Microsoft Download Center. Online documentation for Windows XP SP3, such as Microsoft Knowledge Base articles and the Microsoft TechNet Windows XP TechCenter, will be updated then. For customers who use Windows XP at home, Windows XP SP3 Automatic Update distribution for users at home will begin in early summer.
Thanks to everyone here who installed the public betas – you not only gave us detailed feedback but also helped each other out with timely troubleshooting. Through the beta program we found several important issues and were able to confirm some essential fixes. We couldn’t have done this without you.
We will still be monitoring this forum during the next few weeks in case you have more feedback about the release of Windows XP SP3."