According to Chris Keroack on the Microsoft TechNet forums Windows XP Service Pack 3 was Released to Manufacturing 21 Apr 21, 2008.
It appears that SP3 will be released for download on April 29th via Windows Update and the Microsoft Download Center. Automatic Update distribution for home users is set to begin in early summer.
Great news. A freshly imaged Windows XP SP2 computer is currently installing 92 updates from the internal WSUS servers. SP3 is just in time for a large roll out of new computers around here.