Apple hosted an event today where they revealed iPhone OS 4 to the public. Steve Jobs and others talked about what new features will be available when it is released. The update won’t be available until this summer, but developers will be able to download a preview build today. At the event, Apple highlighted seven new features that will be available which they called “tentpole” features. These features included: Multitasking, Folders, Enhanced Mail, Enterprise, Game Center, and Mobile Advertising. They also announced that certain ones, like multitasking, won’t be available on older devices such as the iPhone 3G and 2nd gen iPod Touch. Oddly they also said the iPad update won’t be available until sometime in the “Fall.” Check out all the new features they showed off below.

They started with the biggest announcement, Multitasking. This has been a long standing rumor that many different sites have been talking about. Apple’s implementation is quite simple, you simply double tap the home button at any time and a bar appears at the bottom showing all the currently running apps. You can switch between them quickly just by tapping the app you want. To achieve this without killing battery life, Apple has laid out six new multitasking services for developers to make use of.

Background audio allows apps to keep running audio in the background. They demoed Pandora running, when they left the app, the music kept playing, and they could even control it using buttons on the lock screen which would normally control the iPod app. The next service is VoIP, which allows apps like Skype to use certain key interface elements that are normally used during phone calls. When on a call you get a similar “double high” status bar at the top of the screen, and when an incoming call arrives you get a notification giving the opportunity to answer. Background location keeps GPS navigation apps, like TomTom, running in the background, so you can still hear turn-by-turn voice directions. Social networking apps like Loopt can also make use of this feature. Local push notifications are available so that developers do not have to rely on Apple’s servers to provide notifications to users. Task completion allows apps to finish running the background, so if you are uploading a photo to Flickr, it won’t be interrupted if you switch to another application. The last service they mentioned was called Fast App Switching. This means applications will instantly resume exactly where you left it when you switch between them.

After a long presentation on multitasking, Apple demonstrated a new way to organize applications by using folders. On the home screen you can now create a folder which contains several apps. The OS will even intelligently name after the category of apps contained in the folder. You can even place folders on the dock for quick access. Following the customization note, they showed that you can now change your home screen wallpaper, just like you can on the iPad.

Currently the Mail application displays each mail account as separate inboxes. This is no longer the case in the new OS. You get the ability to have a unified inbox so that all your new mail appears in one folder. Another change in Mail is the ability to keep your conversations organized with the new “Organize mail by thread” feature. Also, attachments can now be opened in third party apps.

iBooks is coming to all devices. It is no longer an app that is limited to the iPad. You can sync all your books, including the current page you are on as well as bookmarks. This makes Apple more competitive with Amazon Kindle, which is available on a large number of platforms.

To finish off, Apple showed off Game Center and their new Mobile Advertising platform. With Game Center they are creating a social gaming network that is similar to XBOX Live. Game developers can add achievements, leader boards, matchmaking, and friends lists. Many similar networks already exist for games, but none of them are built into the OS like Apple’s. Finally, Apple displayed their new Mobile Advertising platform, dubbed iAd, which provides app developers with an advertisement service they can use in their apps. Apple will sell and host the ads themselves, and give 60% of the revenue to the developer. They want to replace the poorly designed ads that appear in some free applications. To do this they provide interactive content, almost like an app in itself, which runs when you tap on an ad. Advertisements can even make use of location data to provide a map of locations where you can find the product. There is even the ability to buy apps from within ads, all without ever leaving the current application you are in.

This is not everything new that will be revealed in iPhone OS 4, but so far it looks like a huge improvement over the current version. It is a bit disappointing that older devices lack multitasking capability, but I suppose Apple has certain hardware requirements for it to work smoothly. Another surprise is the fact that the iPad won’t get iPhone OS 4 until sometime after other devices. This could be because of testing, or it could be something else. Apple is rumored to release a new iPhone, called the iPhone HD, sometime this summer, perhaps even the same day iPhone OS 4 is released. You can expect a lot more changes to the OS to be revealed, this isn’t over yet.

Update: The first iPhone OS 4 video can be seen below, demonstrating multitasking.
Update 2: Apple has just released the complete video from the event, it is available here.