GamingNexus received confirmation from Microsoft this morning that Live integration WILL
be coming to Microsoft XP. After the Cliffy B mention and the numerous references in the PR, it seemed likely.
A software patch appears to be the most likely delivery method
In addition, DirectX 10 will also be supported by XP, something that was Vista only until this point.
EDIT: GamingNexus has now received conflicting information on whether Windows XP will have DirectX 10 or not. We are attempting to re-confirm with Microsoft that DirectX 10 will be coming to XP.
Live is coming to XP, Direct X 10 is not. Sorry for the confusion but I re-verified everything today and my sleep addled brain got actual releases and rumors mixed up. Also XP will get LIVE the same way that Vista games do, directly through the games and not through an update to the OS.