"Many interesting bits of information came out of the much-anticipated iPhone OS 3.0 event on March 17th, but the one announcement that wasn't expected, the one that will forever change the way the App Store operates is in-app purchases.
In-app purchases will allow developers to charge for additional content for already distributed applications. In its current state, the App Store doesn't allow developers to create and distribute additional content for applications without issuing an application update. For example, if a developer wanted to distribute a new course for a racing game, they need to issue an update for the application to get it into your hands. There is nothing wrong with this system necessarily, but developers lack the ability to charge for that additional content. The only way for a developer to get paid is to release an entirely different application, like a sequel to a game, making it a hassle for you and the developer. In-app purchases will allow developers to be compensated additionally for their extra work while allowing you to decide whether or not you even want that additional content."