Doing this the apps will appear as if they have been purchased from the app store and even presumably get update notifications. Apparently the hack only works for apps that don't fully implement Apple's recommended verification techniques, so it’s up to developers to take make sure their software isn’t susceptible to this simple trick. Apple could also test for this in its application approval process to avoid such issues.
Meanwhile a group known as Hackulous, which has previously broken the copy protection systems used by the iPhone and iPad, says it has developed a program called Kickback that can break the protection on any piece of App Store software. They don’t plan to release their work until at least next month when the store's been established, because they “don't want to devalue applications and frustrate developers.”