Previewed at WWDC, launching in the fall.
Apple to Allow Background Tasks on iPhone?
While Apple had originally set a September 2008 deadline for the service, Apple removed the feature from beta versions in August and has made no other mention of the service. Steve Jobs is reported to have told one customer that the feature was "running a bit late" and that they wanted to get it 100% right the first time.
Apple does not currently allow 3rd party iPhone applications to run as background processes. Instead, apps must shut down completely after use. Push notifications was Apple's solution to allow applications to receive notices while they are not the active application. Certain services, such as Instant Messaging, would benefit greatly from this feature.
By now, it's clear that something is holding up the Push notification service. While we aren't sure what the specific issues are, we've heard that as an alternative Apple is considering allowing apps to run as user selectable background processes. If so, this feature would likely come in the rumored iPhone 3.0 software update but would be limited to only one or two processes on current hardware. The next generation iPhone, however, would likely see less restricted background process support due to its improved hardware.