At WWDC, Apple described a new method for applications to receive notifications even when not currently running. This functionality is required since iPhone applications are not allowed to run in the background. A common reason for this need would be to notify the user of a new message or alert from an application such as instant messaging. Apple's own Mail and SMS applications already benefit from this functionality.
Steve Jobs is said to have emailed a customer who inquired about the status of this new feature. Jobs reportedly said "We're running a bit late. We want to get it 100% right the first time."