Today was the day of Apple’s iPhone OS 4 event, and we’re going to tell you everything you need to know about the next version of iPhone OS.

New APIs

There are over 1500 new APIs available for use by developers in 4.0. Developers can now access features such as the Calendar and Photo Library.

A new framework called Accelerate was also demoed, for hardware-accelerated math functions.

Features

In addition to the API, there are over 100 new features in iPhone OS 4, including Bluetooth keyboards, homescreen wallpapers, camera focus, and multitasking.

Multitasking was discussed a lot at the event, and Apple seems to have added multitasking in a way that won’t drain battery life as badly. Users can now, for example, listen to Pandora while they’re reading mail or surfing the web. Multitasking looks like a second dock on the screen. Speaking of the dock, it’s gotten a refresh from iPhone OS 3.2.

Also, here’s a cool feature: Pandora can be controlled from the lock screen as if it were your iPod. You can stop music, start music, and see what song you were listening to on Pandora.

VoIP also gets a nice integration with the UI. You can run programs like Skype in the background, and get calls even if you were in another app. When you get calls, you are not required to have Skype running in the foreground to use it. So you can receive calls from an employee who’s lost on the road, and open Maps or TomTom to help him find his way.

Push notifications

Push also gets quite a big upgrade: developers are no longer required to use Apple’s push servers. Instead, they can provide their own.

Folders

Users can now organise their apps into folders, just like the app Categories for jailbroken phones. Using this feature, you can organise your games into a folder called Games, or your music apps into a folder called Music.

Unified inbox

Instead of having 12 inboxes for 12 different mail accounts, you can now unite all of those emails into one inbox. Also, you can now have more than one Exchange account. If users want to focus on one account, they can enable fast inbox switching.

iBooks

iBooks will be available on 4.0, and it’ll use the same UI that the iPad uses, but on a much smaller scale. The app comes with a “Winnie the Pooh” book for free. You can also sync your books wirelessly between platforms.

Game Center

Game Center is a bit like Xbox LIVE for iPhone. It’s a place where users can exchanges scores, achievements, and make friends. A bit like “social networking” for games.

iAd

iAd is Apple’s new mobile advertising platform. Instead of going with the traditional “show banners in games, maybe users will click them, maybe not”, Apple has added “emotion” into the ads by making them interactive. All activity happens in the app without the user having to leave the app to go to a third party site.

Final

A build of 4.0 is now available for devs, and we can expect 4.0 to be out this summer, following Apple tradition. iPhone 3GS and the iPod Touch 3G will run multitasking, but anything lower than that will not. Support for the iPhone 2G is supposedly being dropped, since it was not mentioned at the event.