Hmm, just read this so looks like I'm wrong:
Unfortunately, iPhone 3.0 software will not support background processing -- a big disappointment. The reason? It decreases battery life and performance, Apple said today. Apple stated that background processing doesn't let the iPhone go to sleep and doesn't let it operate at lowest power. They also said that background processing chews up CPU cycles, so it's slowing down the foreground app. Apple is already working with third party developers, like Meebo, ESPN and Oracle, to incorporate push notifications into their apps.