This is the second beta of the iPhone SDK targeting iPhone OS 2.1, including bug fixes to iPhone OS as well as an early implementation of the Apple Push Notification Service API. This API is not yet integrated with a live push server.
Apple decided not to allow background apps because they believe it would quickly bog down the phone's processor and eat away at the battery. The company's solution would be its Push Notification Service, which would send all of these applications' transmissions through an Apple server.
Through this, applications will be able to push alerts to the phone through an Apple server, which has a persistent background connection. The company didn't say when the 2.1 firmware will be released, but the push service is slated to roll out in September.
Apple has a decent argument -- as multiple running apps can make many smartphones nearly unusable -- and this is an elegant solution. But you still have to miss a true background instant messaging or streaming radio application. I liked Alexander Wolfe's suggestion of issuing certifications for certain apps to run in the background.
More than that, I wish Apple could trust and empower iPhone users and developers by giving up just a bit of control. Of course, it wouldn't be Apple if it did.