A couple of days ago, Microsoft announced that they are going to offer similar downgrade rights for Windows 7 as what they currently offer for Windows Vista. As it stands right now, if you buy a new computer that has Vista preinstalled, you have the right to downgrade to Windows XP. Anyone buying a computer that comes preloaded with Windows 7 will have the rights to downgrade to either Windows XP or to Vista.
Incidentally, Windows XP’s mainstream support ends on April 14th, and only extended support will be available. From that point on, Microsoft plans to only release security related updates for Windows XP.