Snippet from the announcement:
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.
That's the build number you get in About box/Winver. The real build number is the one you get in a file's properties. Like XPSP1 had 2600.1106 and XPSP3 had 2600.2180. And for XPSP3, we had builds 3311 and 5508 so it must be a number higher than that.