Tech Blender points to this rumor that Longhorn will require 64-bit hardware. You know, one of the fairly easy lessons to draw from the 16- to 32-bit change is that it happened too late, that there was a great deal of pain because everyone tried to live with side-by-side APIs and “sort of” 32-bit versions of Windows etc. So, while I’d not heard this rumor before, it’s not inconceivable that it’s true.