iPhone OS 4.0 is now official with over 100 new features

Multitasking

iPhone OS 4 delivers seven new multitasking services that allow your apps to perform tasks in the background while preserving battery life and performance. These multitasking services include:

Background audio - Allows your app to play audio continuously. So customers can listen to your app while they surf the web, play games, and more.
Voice over IP - Your VoIP apps can now be even better. Users can now receive VoIP calls and have conversations while using another app. Your users can even receive calls when their phones are locked in their pocket.
Background location - Navigation apps can now continue to guide users who are listening to their iPods, or using other apps. iPhone OS 4 also provides a new and battery efficient way to monitor location when users move between cell towers. This is a great way for your social networking apps to keep track of users and their friends' locations.
Push notifications - Receive alerts from your remote servers even when your app isn't running.
Local notifications - Your app can now alert users of scheduled events and alarms in the background, no servers required.
Task finishing - If your app is in mid-task when your customer leaves it, the app can now keep running to finish the task.
Fast app switching - All developers should take advantage of this. This will allow users to leave your app and come right back to where they were when they left - no more having to reload the app.

iAd mobile advertising solution

The final "tentpole" of the new iPhone OS is a new mobile advertising system for the iPhone, iPad and the iPod touch that called iAd.

iAd will allows developers to integrate ads into their apps so that they can possibly release free, ad-sponsored versions of their apps.

Game Center

The iPhone OS 4 beta software and SDK includes a developer preview of the GameKit APIs that allows you to create apps that will work with Game Center, Apple's new social gaming network. Your users can invite friends to play a game, start a multiplayer game through matchmaking, track their achievements, and compare their high scores on a leader board. The APIs are available to you now and Game Center will be available to iPhone and iPod touch users later this year.

Improved email

The Email app on the iPhone now will offer a common inbox for all email accounts setup on the phone. No more flicking back and forth. And corporate users will be glad they can have more than one Exchange account setup. Fast inbox switching is also available now along with threaded email conversations. And you can also open attachments with an application you've downloaded from the AppStore.

iBooks

The iBooks e-book reader you've probably seen demoed on the iPad will now be available to the iPhone as well along with the iBookStore and free copy of Winnie The Pooh (not kidding!).

New features for enterprise

Targeting the enterprise clients, Apple has introduced a bunch of new stuff such as better data protection, Exchange Server 2010 support, SSL VPN support, and mobile device management.