The iPhone Push Notification Service: what to watch out for

This week Apple introduced its Push Notification Service API to a restricted set of developers.
What's so special about this? The new iphone 3G does not allow developers create applications that can run in the background. An example would be a chat application for Facebook that sounds a 'ping' when one of your contacts goes online; this chat application would need to be running all the time on your iphone and now and then ask the server "anybody new?". Or it just has to sit listening for a message from the server telling that one of your contacts logged on.
This push notification service sits between the 3rd party server (e.g the facebook application server) and the iphone device. See here for a clear basic architecture picture. It allows any 3rd party server to contact this Apple(!) service, which in turn then contacts the related iphone device. This means Apple will have all the knowledge of all the 3rd party applications and their communication with the connected iphones!! Scary. This sounds sooo Microsoft.
Here's another post dedicated to this too.