As you recall, Apple announced a unified Push notification service for developers at the WWDC in June.
"Push notification" is just a fancy name for apps having the ability to receive background notifications. So in a way, this is Apple’s answer to having some background functionality for iPhone applications. Applications which require an Internet connection will benefit the most from push notification.
It appears that the API for the push service has been released to a handful of developers, so watch out for the September release for some new functionality on your iPhone’s apps.
We’re particularly looking forward to being able to receive IM’s without having to keep the IM app open at all times. In other words, say hello to free text messaging.