Microsoft Deployment Toolkit includes a check in the Lite Touch script LTISysprep.wsf to ensure that you are using the right version of SYSPREP when preparing your Windows XP image for capture.  With the release of Windows XP SP3, that means you need to have updated SYSPREP files too.  If you integrate the service pack into Windows XP SP2, you'll get the latest OS files, but you won't get the latest SYSPREP files - the SUPPORT\TOOLS\DEPLOY.CAB will not be updated.

So, that leads to the question:  Where get I get an updated DEPLOY.CAB with the right SYSPREP files?  Simple, download one of these:

Windows XP Service Pack 3 Deployment Tools (just the DEPLOY.CAB)
http://www.microsoft.com/downloads/details.aspx?familyid=673a1019-8e3e-4be0-ac31-70dd21b5afa7&displaylang=en

Windows XP Service Pack 3 - ISO-9660 CD Image File (the DEPLOY.CAB plus the rest of the SP3 content)
http://www.microsoft.com/downloads/details.aspx?FamilyID=2fcde6ce-b5fb-4488-8c50-fe22559d164e&DisplayLang=en

If you donwload the ISO, you'll need to either burn it to a CD or mount it using an appropriate tool.  Then you can get to the DEPLOY.CAB in the SUPPORT\TOOLS\DEPLOY.CAB folder. 

Once you have the DEPLOY.CAB file, overlay the SUPPORT\TOOLS\DEPLOY.CAB in your XP SP3 source directory (e.g. \Distribution\Operating Systems\XPSP3\SUPPORT\TOOLS\DEPLOY.CAB) with this new version.  (If you have extracted the SYSPREP file and placed them in the OS source directory, you'll need to repeat that process with the SP3 versions of the files, or just remove the extracted SYSPREP files and let the LTISysprep.wsf script take care of it for you as part of the capture process.)

(Updated to include a link directly to the DEPLOY.CAB download, posted today.  Thanks to Blake Handler, http://bhandler.spaces.live.com/, for pointing that out.)