As Apple announced briefly during WWDC, the Push Notification API has been released to a handful of devs, according to CrunchGear. The Push Notification service allows 3rd-party iPhone applications to stay connected to servers which allow notifications to be displayed when new information shows up. This is akin to Apple’s iPhone apps, which display an icon badge or text alert, or sound an alarm when new information is received. In version 2.0 of the iPhone’s firmware, third party applications cannot run fully in the background, but certain apps (such as AIM) stay connected. However, no notifications of new information (such as new IMs) are displayed.
Apparently, the 2.1 version of the firmware will have Push Notification availability for all third party apps.