Microsoft has apparently pushed back the scheduled release date of SP3 to the second half of 2007. Microsoft executive Steve Ballmer had originally announced that the 3rd service pack for XP would be released before the launch of Longhorn (now Windows Vista).
Bink suggests that we could see a "Rollup Package" coming from Microsoft in the near future for XP, with the possibility of that package replacing SP3 all together.
One can also reason that Microsoft will use the delay of SP3 to further the cause of upgrading to Vista later on this year.