Microsoft has made Windows XP Service Pack 3 Release Candidate 2 (RC2) available to the public via its Download Center:

Brief Description
Windows XP Service Pack 3 Release Candidate 2 is available to the public. Specific registry settings will allow you to be offered SP3 via Windows Update.

Instructions
Download the script and run it on a machine currently running Windows XP Service Pack 2. The script sets a registry key on your system. The registry key is required for Windows Update to recognize your machine as a valid target for Windows XP Service Pack 3 RC2.

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.

It is recommended that you apply the resulting update package to an activated, genuine copy of Windows XP, in a test environment. As with any pre-release software, it is also recommended that you back up files and settings on your machine before applying this update package.

Interesting. This is the version Microsoft shipped to testers about a week and a half ago and is, I would guess, very close to what will be the final code.