iOS 4, previously known as iPhone OS 4, is a major update to Apple's mobile OS which brings a handful of significant changes—namely Apple's implementation of "multitasking" plus the opening up of thousands of APIs to third-party developers—while the rest of the improvements are basically tweaks to existing functionality.

We'll say up front that we like the update. For iPhone 4, iPhone 3GS, and iPhone 3G users (as well as second- and third-generation iPod touch users), iOS 4 will add useful functionality that will make your device more useable than ever. There are, however, some obvious downsides, and we'll address those in this review.

Because iOS 4 is launching ahead of the new iPhone 4 (and it runs on more devices than just the new iPhone), we're reviewing it separately from the phone itself. There is some functionality that is specific to the iPhone 4, which we'll address in that review when it comes out. For the purposes of this review, though, we used iOS 4 on an iPhone 3GS—the most current iPhone available ahead of the iPhone 4 launch.

Multitasking

The biggest and most obvious update to iOS 4 is the ability for third-party applications to "multitask"—that is, you don't have to terminate them in order to do something else in another app. This is functionality that has been requested of Apple since the launch of the original iPhone in 2007, and it has been a long time coming. (Only the iPhone 4, iPhone 3GS, and third-gen iPod touch get multitasking.)

Apple's own apps have always been able to truly multitask (you can get iCal alerts while playing a game, for example, or listen to music from the iPod app while reading a webpage), but third parties can now take advantage of a limited set of multitasking capabilities in order to make things easier on the user.

Why "limited"? Because iOS multitasking isn't really multitasking in the traditional sense—it's certainly not what you get on a desktop computer, or even what you get from Apple's own iPhone apps. Apple claims that it only allows for certain functionality so that the OS can continue to preserve battery life and performance in a sane manner. So, what exactly can you do with this new feature? Really, the answer isn't so much that apps will be "multitasking," but rather that they'll be "doing a few things in the background":

Audio : you can now listen to streaming music from apps, like Pandora or newscasts through the NPR app, while doing other activities on your phone. Previously, you would have to quit out of the app (and therefore stop your music stream) if you wanted to respond to an SMS or read your e-mail, and now that's no longer the case. Hallelujah.

: you can now listen to streaming music from apps, like Pandora or newscasts through the NPR app, while doing other activities on your phone. Previously, you would have to quit out of the app (and therefore stop your music stream) if you wanted to respond to an SMS or read your e-mail, and now that's no longer the case. Hallelujah. VoIP : similarly, you can carry on Voice Over IP calls on services like Skype without having to quit the app if you need to perform other tasks.

: similarly, you can carry on Voice Over IP calls on services like Skype without having to quit the app if you need to perform other tasks. Location : apps that need to poll your location, such as GPS and direction apps, will be able to do so in the background. No longer will you need to keep the app in the foreground just so it can keep track of where you are.

: apps that need to poll your location, such as GPS and direction apps, will be able to do so in the background. No longer will you need to keep the app in the foreground just so it can keep track of where you are. Local notifications : third-party apps no longer have to rely solely on push notifications if they want to alert you of something on your phone. If you have an alert set in, say, one of Omni's applications, it can ping you when the time comes instead of having to go through a convoluted series of Internet tubes to get to you. This, of course, reduces your reliance on an Internet connection to get certain types of alerts and helps cut down on overall wireless bandwidth.

: third-party apps no longer have to rely solely on push notifications if they want to alert you of something on your phone. If you have an alert set in, say, one of Omni's applications, it can ping you when the time comes instead of having to go through a convoluted series of Internet tubes to get to you. This, of course, reduces your reliance on an Internet connection to get certain types of alerts and helps cut down on overall wireless bandwidth. Complete tasks : if you start a task in an application and then switch to another one (such as downloading a new map in your favorite game), it can now complete the task in the background instead of forcing you to sit there and wait on it.

: if you start a task in an application and then switch to another one (such as downloading a new map in your favorite game), it can now complete the task in the background instead of forcing you to sit there and wait on it. Fast app switching: this is basically "pausing" an app where it is, which allows you to quickly switch away from it and then switch back, picking up where you left off.

What can't your third-party apps do while in the background?

Grab new updates : those of you who were hoping that your Twitter, IM, or IRC client would pull down updates while hanging out in the background will be disappointed. Unless those apps make use of push notifications to alert you of new messages (as the AIM app does), apps won't be able to check for updates on their own unless they're in the foreground.

: those of you who were hoping that your Twitter, IM, or IRC client would pull down updates while hanging out in the background will be disappointed. Unless those apps make use of push notifications to alert you of new messages (as the AIM app does), apps won't be able to check for updates on their own unless they're in the foreground. Work across the entire OS: users have long hoped that some of their favorite apps (such as TextExpander) would be able to work in all parts of the OS, such as Mail and Messages, but that won't be the case. This isn't Mac OS X here—Apple still wants to keep each app to itself for the most part.

Now that we've told you what the new feature can and can't do, we'll tell you how to use it. Anytime you launch an app and then switch to another app, the first app is automatically "backgrounded" and added to your app drawer. You can access this drawer from anywhere by hitting the Home button twice, which will slide your screen up and show you a row of icons that you can swipe through.

If you swipe right immediately after hitting the Home button twice, you'll always get the orientation lock (more on this later) as well as your audio controls for the app that's currently playing audio. If you swipe right, you'll just keep getting apps that you recently used—tap any of the app icons to switch back to that app, and it will pick up exactly where you were before you switched away. Although the multitasking feature is limited overall, the ability for the OS to keep your spot in whatever you were doing is certainly welcome. (It's worth noting that many developers have put in good effort to save your state within their apps in order to replicate this on their own.)

One odd thing we noticed about the app drawer is that it seems to stay in portrait mode when you double tap the Home button while in landscape:

The drawer itself coming up on the side (instead of the bottom) isn't so much the problem, but the app icons are still rotated as if the phone were in portrait. Someone couldn't at least rotate the icons so they're facing the right way when viewing it in landscape? This seems like an oversight that is atypical of Apple's usual attention to detail.

And, although Steve Jobs trashed the idea that users should be managing apps on their own (he said something along the lines of "If you have a task manager, you have failed!" at a media event in April), you can quit the currently running apps if you want to. You shouldn't have to—Apple basically freezes all their processes in the background unless they're performing one of the aforementioned kosher functions, so they shouldn't be sucking up any of your performance—but it makes those of us with neatness neuroses happy.

Just tap and hold on the app that you want to close in the app drawer, and they'll all begin to wiggle (just like if you were to tap-and-hold on the home screen to move an app around). Tap the minus sign in the upper left-hand corner and the app will disappear.