Apple released the iPhone OS 4 upgrade, or iOS 4, for all iPhone and iPod Touch users who had their devices hooked to iTunes this week.

The update weighs in at 378MB, and you need the latest version of iTunes (9.2) to be able to download iOS 4.

Unfortunately, if you have an original iPhone or iPod Touch, your best bet to run iOS 4 is to buy a new iPhone. iOS 4 works with iPhone 3GS, iPhone 3G, and second- and third-generation iPod Touch devices. Take note, however, that not all of its features are compatible with all devices. For example, multitasking is available only with iPhone 4 and iPhone 3GS.

Here's a closer look at some of the highlights of the latest version of the Apple iPhone OS.

Wallpapers for the home screen

Though it isn't a life-changing feature, the fact that you now can select a wallpaper for your iPhone's lock and home screens is pretty exciting. Installing iOS 4 on an iPhone 3GS takes under 10 minutes, and you are also prompted to do a quick carrier-settings update. The device then restarts, restores all your data (music, photos, contacts, email accounts, and so on), and presents you with your new home screen, which can now feature a background image.

You can choose two different images for your lock and home screens, or use the same one throughout. Note that the Calculator icon has been refreshed as well (though the app itself remains the same).

Multitasking is a joy

For many iPhone owners, multitasking was the biggest missing feature. iOS 4 aims to solve this problem. In iOS 4, tapping the Home button to close an app now only hides the app's activity. To see your open apps, you simply double-tap the Home button - the current screen pushes up to reveal a dock. You can swipe left or right to browse through the open apps. To close an app or task, you tap-hold on an app.

If you are reading email and you want to listen to Pandora in the background, for instance, you tap the Home button once, open Pandora, and then double-tap the Home button and tap on Mail to return where you left off.

The problem right now is that the majority of apps are not multitasking-enabled, but in the coming weeks you can expect developers to update the bulk of these apps to add multitasking support.

NEXT PAGE: Unified Inbox and threaded email

  1. There's more to the OS upgrade than just multitasking
  2. Unified Inbox and threaded email
  3. Organise your apps in folders
  4. Improvements to the messages app
  5.  Digital zoom and geo-location for your camera