Apple's Push Notification Service is getting closer to a reality; the first implementation of it was included in a beta release of iPhone Software 2.1, which was seeded to iPhone developers yesterday. Once the service is live, app developers will be able to send various notifications to their users even if the app in question isn't running on that user's phone.
When Scott Forstall explained the iPhone 2.0 software, first to the media last March and later to developers at June's Worldwide Develolpers Conference, he made reference to Apple's Push Notification Service. PNS will allow developers to send notifications to users' iPhones even though their apps weren't running at the time. It's designed to be a workaround for the fact that Apple doesn't allow iPhone apps to run in the background.
For example, if you're not using the AIM app, you won't see any new instant messages until you open that app. But once Push Notification Service is rolled out, AIM's can send a notification of your new messages to Apple's "cloud," and then Apple will push it down to your phone. Forstall mentioned pop-up text-based notifications (like those used currently by the Calendar app), "badge" notifications (like the red circle icon on the that shows you how many unread Mail messages you have), and sounds.
Apple intentionally made sure that iPhone and iPod touch applications can't run as background processes, so users know when they quit an app, it really quit. Running background processes also saps battery life and slows down processor performance. The PNS uses Apple's always-on IP connection to the iPhone, which it "loans" to developers to let them accomplish roughly the same objectives. The final service is expected to roll out sometime in September.
