I have been using the push notification service from Urban Airship for a while. It is a great solution if you do not want the overhead of running your own server infrastructure and having to interface directly with Apple's service.
Today they announced something called AirMail which seems to be a push notification on steroids. Some of the interesting new features that caught my eye:
- a persistent inbox in your app allows the notifications to be stored so the user can go back and review them later. This seems to be a big gap in the current iPhone OS that I have to hope will get fixed in OS 4.0. Right now it is easy for a user to miss a notification if several arrive at the same and once the user closes the notification there is no way to ever get it back.
- delivery receipts - get a confirmation that the notification was delivered and that the user actually read it.
- Include rich text and media in the push notification. I have not seen how this is implemented but I assume the push notification actually contains a URL pointing back to the actual message hosted on a server.