Apple announced iPhone OS 4 Summer..

What's new in iPhone OS 4

100+ New User Features
With iPhone OS 4, iPhone owners will enjoy over 100 new features that make it easier to interact with apps, manage email, read books, and more. New features include the following:


Multitasking
Developers will have access to seven multitasking services, which will allow tasks to be performed in the background while preserving performance and battery life. So you’ll be able to make a Voice over IP call while playing a game or checking email, find a restaurant on Urbanspoon while listening to Pandora, and more.

Folders
Organize apps into folders with drag-and-drop simplicity. Get faster access to your favorites and browse and manage up to 2160 apps at once — many more than the current limit of 180 apps.

Even Better Mail
See messages from all your email accounts displayed together in a unified inbox, switch between inboxes more quickly, organize messages by threads, and even open attachments in third-party apps.

iBooks
The best way to browse, buy, and read books on a mobile device. Easily flip through the pages of a book you've downloaded from the iBookstore while listening to your music collection. Browse tens of thousands of books — many of them free — by title, author, or genre. You can even check out reviews and read sample pages before you buy.

Developers
With a rich set of over 1500 new APIs, the iPhone SDK 4 provides developers with a broad range of technologies to enhance the functionality of their iPhone and iPod touch applications. Developers can use these new APIs to make their apps more powerful, more feature rich, and more amazing than before. They can even use new GameKit APIs to develop for Game Center, the social gaming network Apple will launch later this year.
Learn more about the iPhone Developer Program
and iPhone SDK 4
iAd

iAd is a breakthrough mobile advertising platform from Apple. With it, apps can feature rich media ads that combine the emotion of TV with the interactivity of the web. For developers, it means a new, easy-to-implement source of revenue. For advertisers, it creates a new media outlet that offers consumers highly targeted information.
Enterprise

iPhone OS 4 offers more useful features for businesses. With enhancements to security, scalability, and compatibility, IT managers have even more reasons to deploy iPhone throughout their enterprises.