Since WinFX is making its way to Windows XP, that is somewhat feasable. But a lot of Longhorn APIs won't be backported into Windows XP. Longhorn will feature a new version of IE and DirectX, and NGSCB integrated into the system that might not be fully backported, so you might not be able to run certain things on older versions of Windows...

Personally I think we might see warez groups start releasing service packs for Windows.