The number of questions we've seen regarding this issue indicates that there is still some confusion out there... So, I'm going to interupt the WCS gamut mapping series to clarify things: WCS will not be available for any pre-Windows Vista OS platforms. We have no plans to port WCS to Windows XP or any other pre-Vista Windows version.
I think the confusion around this stems from the fact that both the Windows Presentation Foundation (WPF, formerly WinFX), and the XML Paper Specification (XPS, formerly Metro) will be made available down-level.
The fact that WCS will not be available on Windows XP or Windows Server 2003 was one of the main reasons we decided to wrap WCS profiles in an approximating ICC profile for embedding purposes. This ensures that some reasonable color management will be performed using the ICC profile on down-level platforms. Even when printing from Windows Vista, there is the possibility that a network printer may reside on a server running a down-level OS version.