This is what the Vista EULA found here (PDF) says
13. UPGRADES. To use upgrade software, you must first be licensed for the software that is eligiblefor the upgrade. Upon upgrade, this agreement takes the place of the agreement for the software you upgraded from. After you upgrade, you may no longer use the software you upgraded from.So there you have it. You've bought a copy of Windows XP, you buy a copy of Vista to upgrade and it automatically makes your XP key inavlid. Interesting take on property rights from Microsoft on that one it seems. If you do it you better hope you don't want to rollback.