Thanks to a "friend" ;) for pointing me in the direction of this story
Dear Beta Tester:
We have released an updated build of the Update Rollup 1 for the Windows Update V4. You must use Windows XP RTM and Windows XP SP1 to participate. If your computer is not running either operating system, please disregard this mail. The Windows Update Site customer experience is very important to Microsoft; we believe the input you give us from your beta testing has helped and will continue to help us release an update package that represents the best of what Microsoft offers.
This package, Update Rollup 1 for Microsoft Windows XP (KB826939) is being released to the Windows Update beta site in two versions: Full (AFPBeta) and Express (AEPBeta). Uninstall the previous version from Add/Remove Program and then install this updated version is required. The Full version will be offered through Windows Update Catalog site, Windows Update Consumer site, and through SUS for both Windows XP RTM and SP1. The Full version will be listed on the Consumer site for detection test for beta testing only. As a result, you will see two similar items with the same title but a different download size - the full and the express version.
The Express version is offered only through Windows Update Consumer site (scan for updates) and only for Windows XP SP1 only. The purpose of the Express update is to provide a small and efficient download for Windows XP SP1 users who want to get fully up to date with critical updates. This update consists of 22 critical and security updates for Windows XP released since Windows XP SP1 originally shipped, rolled into one convenient package. Installing this item provides you the same benefits as installing the individual updates. Our goal is to evaluate the performance of detection and installation segments of Update Rollup. We will focus on verification before and verification after the installation of Update Rollup package.
Thank you very much!
Windows Update Beta Program
This means it will still be at least two weeks before End users will receive this update