The biggest news today. Apple has officially announced the iPhone OS 4.0 with over 100 new features. Here are some of the great features:

1. Multi-tasking – oh yes! finally! Apple finally put on the multi-tasking feature to the iPhone. 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. But the feature will work on iPhone 3GS and iPod Touch 3rd gen only.

Check out the rest after the break.

Press release here.

2. Folders – Tired of sliding those pages? Now you can group those apps in a folder.

3. 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.

4. iBooks – Like the iPad. 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.

5. 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.

6. 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.

7. Game center – Nice one here, now you can play game online like the Xbox live. Invite friends for online multiplay.

iPhone OS 4 will ship this summer for iPhone and iPod touch, and for iPad in the fall. Probably together with the new 4th gen iPhone.

Here are some shots from Engadget at the keynote: