Something significant has obviously delayed the original September launch of push notifications, Apple's solution to not allowing apps to run in the background. MacRumors hears that Apple is considering allowing real background processes instead.
Android and especially the Pre have made background apps and the true multitasking they allow look a lot sexier, and the iPhone's one-app-at-a-time paradigm more restrictive, even with the innovative compromise of push notifications.
Push notifications, for the uninitiated, would allow apps like AIM to send you notifications (through Apple) of say, new IMs via an SMS-like prompt, even while the application isn't running. So you could kind of think of the app as running in the cloud, essentially. Not multitasking by any means, but for some an acceptable compromise on battery-draining background apps.
MacRumors says that if what they're hearing is true, and Apple allows apps to run in the background, it would happen with the iPhone 3.0 software. On the current iPhones, it would likely be restricted to one or two processes at a time, but with the beefier hardware of the next-gen iPhone, it'd be less restricted.
Having apps actually run in the background might actually be worth the longer wait. [MacRumors]