Microsoft has confirmed to news site The Register that Windows 7 will offer a downgrade to Windows Vista and Windows XP for up to 6 months after general availability. The news page seems to suggest that the downgrade will be available for Windows 7 Pro and Windows 7 Ultimate.
I would guess that the downgrade availability would be becuase there are still a large group of people who prefer Windows XP to Windows 7 and Vista.
So, if you still love Windows XP, you don’t need to worry about buying a new computer. Personally, I think Windows 7 will be much better than any previous Windows version, so see what you think before you downgrade.