Microsoft yesterday made Service Pack 3 RC2 available to both testers and the general public. They have provided a small script to run on your XP SP2 machine that will allow for SP3 to be available through Windows Update.
The build, now at v3311, cannot be installed over previous SP3 betas. Versions of Windows XP Service Pack 3 prior to Release Candidate 2 should be removed before attempting to use the registry key. Windows Update will not offer Release Candidate 2 to machines with previous versions of the Windows XP Service Pack 3 beta.