Apple on Thursday sent out an email to registered iPhone developers enticing them begin testing applications using the Apple Push Notification service.
Push Notification System

Start testing your applications using the Apple Push Notification service today. Log in to the iPhone Dev Center and review the Apple Push Notification Programming Guide and Getting Started video.

The Push Notification service is Apple’s answer to background applications on the iPhone. The system which is set to be released alongside iPhone OS 3.0 in the Summer will allow applications that are not currently running to listen for notifications which can be delivered in the form of icon badges, text messages, and alert sounds. For example an instant messaging application will be capable of notifying the user of new instant messages without the application actually running.

Apple claims that running applications in the background would damage battery life of the iPhone, adding that the Push Notification Service will only have a 23-percent hit on battery life when in standby mode.