Apple has posted iPhone SDK for iPhone OS 3.0 beta, registered developers can download it now. (Update: Standard or Enterprise Program members only.)
Here are some of the new features available to developers:
In App Purchase
Allow users to purchase content or services from your application using the Store Kit framework. This new framework handles the financial aspects of the transaction, processes payment with the iTunes Store, and provides your application with information about the purchase.
Apple Push Notification service
The Apple Push Notification service provides a way to alert your users of new information, even when your application isn't running. Send text notifications, trigger audible alerts, or add a numbered badge to your application icon.
Using the new External Accessory framework, your application can now communicate with "Made for iPod" hardware accessories attached to iPhone or iPod touch through either the 30-pin dock connector or wirelessly using Bluetooth.
Peer to Peer Connectivity
Add multi-player capabilities to your games using the peer to peer network connectivity and in game voice communication features of Game Kit. This powerful new framework allows any application, not just games, to communicate between devices using Bluetooth-no pairing required.
You can now embed maps within your applications using the new Map Kit framework. Map Kit works with the Google Mobile Maps Service and features panning and zooming, custom annotations, current location and geocoding.
iPod Library Access
Access music, podcasts, or audio books in a user's iPod library directly from your application using the updated Media Player framework. You can play, repeat and shuffle songs or whole playlists, or create sequences of songs using custom searches.