When you go out to buy a copy of Windows Vista next year, regardless of what version you purchase, you'll receive a DVD that has every edition loaded on it and ready to go. The unlocking mechanism for which components will be installed is going to be the CD key that will either come affixed to your computer case, or attached the the manual. They're calling the system Windows Anytime Upgrade, where users can upgrade from their current version to a more powerful one just by purchasing and inputting a new CD key
Microsoft, when asked about the possibility of the unlock system being hacked, dismissed the idea saying that Vista is their most secure OS yet. Of course, whether or not the system is unhackable remains yet to be seen.