The major change in the UIKit API is that Apple has added separate types for the three notification methods: badge, text alert, and sound. Developers can register their apps for these different notification types individually for the needs of their users. Previously, apps registered to received remote notifications but controlled the type via settings. Developers we spoke to universally agreed that this was a welcome improvement.
Still nothing in the way of non-obtrusive, Google Android- or Palm Pre-style notification, nor any indication of how Apple will gracefully handle multiple modal dialogs popping up all at once, but there's still time to wow us with something awesome come WWDC, right Apple?
Ars also mentions some interesting developments regarding Dashcode (widget development) for iPhone, and more, so be sure to check them out.