This is something we had not heard yet! We did know some of Vista's core functions would be included but to hear both Avalon (GUI) and Indigo (communications) would be included is news to me!

Besides bug and security fixes, SP3 will contain the XP versions of Avalon (GUI) and Indigo (communications). Vista uses a whole new programming development model, which will obsolete a generation of enterprise apps. Think Visual Basic, for example. The XP versions of Avalon and Indigo will allow new enterprise apps to be backwards deployed to the millions of XP clients that will be around enterprises until the next decade, at least.
http://forums.winxpcentral.com/showthread.php?t=15125
WinXPcentral - SP3 will contain the XP versions of Avalon and Indigo!