Obviously, it'll be a while before we can actually see the system in action, but the push notification system promises a modest improvement in the way that we interact with many of our iPhone applications. Instant-messaging programs, for example, will become more useful, since you can get messages without having to be running the application in the foreground (or resorting to workarounds like mail or SMS notification). News apps can now grab your attention when an update comes in. Social networking applications could let you know if you've received messages you might want to read.

But push notifications are hardly a panacea. Applications still can't run in the background, which hurts applications like third-party Internet radio applications, since you can't listen to audio from those apps while performing other tasks on the device. And as nice as notifications will be for instant messaging, in many ways it's a poor substitute for a true background-capable application.

According to Forstall, the reason Apple chose not to do background processes is primarily due to concerns for battery life and standby time--the company's tests showed that background processes gobbled up power like at a hot-dog-eating contest.

There are other usability questions about the notification system, too. For example, there's the matter of notification overload: Getting a text dialog box every time you get a new Twitter update would try the patience of even the most devoted Twitter user. Will the slew of notification-capable applications turn all of our iPhones into incessantly-chiming, dialog-box-spewing annoyances? When I'm on instant messenger, I'm often carrying on a few conversations at the same time; I could see notifications for each of those messages quickly becoming overwhelming.

It seems likely Apple or the third-party developers will provide a way to control the level of notifications on an application-by-application basis, or at least the ability to turn them on and off, but at least a portion of that responsibility will probably fall to the developer. That means all it takes is one poorly thought-out application to ruin your iPhone experience.