One of the features clearly missing from the iPhone SDK (together with calendar and notes access...) is push. There is no push in iPhone, besides MobileMe and ActiveSync. If you are a developer and want to push something to the phone, tough luck. There is no way.
In the last keynote, however, the Apple guys showed us the way. A push mechanism completely managed by Apple, the big brother. You want your application to receive something from the network? You send it to Apple, they send it to your iPhone. It will be called The Apple Push Notification Service.
The idea they explained in the keynote is that you can actually send only the notification, there is no process in background. Your application does not really wake up. You either hear a noise (custom sound), get the icon of your app to change with a number (icon badge) or see a text message (same as when an SMS arrives).
It is not perfect, there is big brother in the middle but I can live with it. In the keynote, they said it will be here by September.
Apparently, they put it in a beta of 2.1 for developers then they pull it from the latest beta... If you were waiting for push in September, too bad. Push is not going to be there, at least until 2.2 is out (they have to give developers some time to play with it).
I suspect the issue is with the MobileMe meltdown. I feel they were planning to use the same channel for it and they decided they were not ready. Or maybe they just decided competition from developers is bad. Better to keep the thing closed for a few more months, while they fix their own product. We are open as long as it does not hurt us... I heard that before...