Blizzard new battle.net system is really the best of both worlds. By "owning" a game in your account, you can then logon to online play using that account from any computer that has the game installed. You simply use the CD key to install and link that CD key to your account. Can't link it? Can't play online. Offline works fine, though - assuming it's not WoW (or their next MMO). But once you've "installed" the game to your bnet account, you have access to play it from any installation - regardless of what CD keys were used to install the game.
It's really a logical extension of old-school CD keys into the age of internet gaming.







