Microsoft is trying to give users an easy upgrade path when it comes to switching versions of Windows Vista. In something they've never done with previous releases of any software of theirs, all versions of Windows Vista will ship on the same disc. What version is actually installed will be based entirely on the key you provide it. The Windows "Anytime Upgrade" feature will let you fork out cash on-demand to get an upgraded version, and viola, it is unlocked from the disc to be installed.
While you might think MS is just asking for trouble doing this, they are quite content that it will be secure from prying eyes. Of course, the money they will say in production and handling likely won't be passed on to you, with them declining comment if this will result in upgrade prices being lowered.