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Computerworld - Like clockwork, Apple has once again offered up numerous improvements to the iPhone's software in a June release. Although all eyes were on the new iPhone 4 at Apple's Worldwide Developers Conference two weeks ago, the new smartphone would be just a stylish chunk of stainless steel and glass without iOS 4, the new operating system that arrives today.

For the last couple of weeks, I've been running Apple's newly minted iOS 4 to find out whether it, like previous OS updates, breathes new life into the iPhone lineup.

While the overall look and feel of the iPhone's operating system remains unchanged, Apple has built in steady and solid improvements that add features users have asked for since the last big update a year ago. Instead of changing things for the sake of changing things, Apple tends to focus each major iOS release on a few well-implemented features. This year's release -- which iPad users will get this fall -- shows that the development teams at Apple have delivered, among other things, limited multitasking for third-party applications, better application sorting and management, and improved e-mail functions. The end result is an updated operating system that makes using the iPhone a much more polished experience, all without sacrificing battery life.

The fourth iteration of the iPhone operating system will run on the iPhone 4 (which arrives on Thursday), last year's iPhone 3GS and, to a lesser extent, the iPhone 3G that arrived in 2008. Recent iPod Touch models are also supported. If you're still using the original iPhone from 2007, or the first iPod Touch, you won't be able to run iOS 4

Although Apple's Web site boasts that iOS 4 offers "100 new features," only a handful of those features will stand out for most users. It's great that you can change your home screen wallpaper, send gift apps to your friends and create custom playlists right on your phone, but those aren't the big improvements to arrive this year. (And while FaceTime video chat got a lot of attention two weeks ago, that's more about the new hardware than the iOS 4 software. I'll have more to say about that when I review the iPhone 4.)

With the focus now on iOS 4, here are the five changes that should persuade you to get the upgrade as soon as you can.

Multitasking

Finally, third-party multitasking has arrived for the iPhone -- at least for the owners of the iPhone 3GS and the new iPhone arriving this week. (Users of the iPhone 3G won't be able to take advantage of the new multitasking services.) And you'll be happy to learn that Apple's implementation of that functionality works pretty well.

True, it's not the full-on multitasking you get in Mac OS X. But Apple has smartly implemented seven separate services that allow applications to continue to perform tasks while the user switches to another application. The reason for the limit? To help preserve battery life.

The multitasking services include the following: background audio, which means users can play music from Pandora in the background while using other apps; background voice-over-IP, so Skype users can remain on their calls -- even if they switch to other apps; background location, which allows apps that use GPS to track where you are; push notifications, which were first introduced in last year's iPhone OS 3; local notifications, a service that can track and alert application events without relying on Apple's push notification servers; task finishing, so uploading photos to Flickr isn't interrupted just because you switch to another app; and fast app switching, which quickly saves the app's current state when you switch to something else -- and brings it right back to where you left off when the app is relaunched.