From Microsoft:
Windows XP SP3 includes previously released updates for the operating system, as well as a small number of new fixes and additions that should not significantly change the Windows XP experience. Windows XP SP3 will be released to the web (RTW) on April 29, 2008. For customers deploying Windows Vista, enhancements to Windows XP SP3 (such as Network Access Protection or NAP) will make it easier to co-manage Windows XP and Windows Vista within the same corporate environment. For further information regarding Windows XP SP3 RTM, please visit the TechNet Forum blog post at http://forums.microsoft.com/TechNet/default.aspx?SiteID=17.
I received the final code recently and will publish a few install shots later today.