There's a funny little meme going on discussing whether or not WoW constitutes a cloud. John Willis asks the question and Markus Klems concludes that it is. (BTW, both of them have excellent blogs that are a must-read for anyone interested in cloud computing).
I argue that although an awesome game, it doesn't really have much to do with cloud computing.
WoW doesn't fit in any of the categories of cloud computing:
Infrastructure-as-a-Service: No. It doesn't provide anyone with infrastructure except for internal use.
Platform-as-a-Service: No. It does not expose it's platform for others to develop applications on. There's the ability to develop very simple game-related macros and add-ons, but that doesn't make it a PaaS.
Software-as-a-Service: Well, you'd be hard-pressed to call anything with a 2-3 GB client a SaaS.
WoW is not really architected as what you would expect of a cloud application. It has no concept of clustering. So while the business can scale in the sense that it can handle millions of concurrent users, it does so in a very rigid way that would not fit the needs of a true web application. What Blizzard has done is partition users into completely siloed servers, what they call "realms". So if you are on the Eldre'thals realm (an actual physical server with that name), you cannot interact with a player on the Agrammar realm (another physical server). Your character wil not be able to chat with, send mail, see or deal in any way with characters on other servers.
Imagine if that were the case in a social networking application, for example. You would only be able to connect with friends that are signed up on the same physical server as you. Absurd, right?
BTW, an alternative way in which Blizzard could have dealt with the scaling issue could have been taking a similar approach to what many of the online gambling and casual gaming applications do -- partition the application based on location. So in a poker game things are partitioned by Tables, not users. Everyone playing at Table 1 is interacting with Server 1. If you move to Table 10, you're now interacting with Server 10. That way all players can interact with one another.
Now, don't get me wrong. I'm not saying that Blizzard made the wrong choice. Their solution is clean and highly scalable, and people love WoW. I'm just saying it's not a cloud app in any way.