This long-awaited feature will finally arrive with iPhone OS 4.0, but it's a much more subtle and careful implementation than most users might expect. Apple is achieving the appearance of multitasking--the ability to run more than one program at a time--through a combination of app-switching features and background processes managed by the operating system itself. What is being added to iPhone OS 4.0 is the more traditional concept of apps running, full bore, whether you can see that or not.

"[Multitasking] is really easy to implement in a way that drains battery life. It's easy to do it in a way that reduces the performance of the foreground app and makes the phone feel sluggish," Steve Jobs told reporters on Thursday. "We've figured out how to do it and avoid these things."

When people complain that the iPhone doesn't do multitasking, that doesn't necessarily mean they're desperate for a bunch of apps to run on their iPhone simultaneously. (Sometimes they do; more on that in a minute.) No, a major productivity hurdle on the iPhone is the fact that sometimes a user needs to use several apps to perform a task--for example, switching among Mail, Safari and Notes while researching a memo--and switching rapidly between apps is just not something the iPhone handles well. You have to press the home button, find the next app among your home screens, then launch that new app, navigate to where you want it to be, and then repeat the process to switch back.

So when Apple says it's added multitasking to iPhone OS 4.0, one of the big things it's added is actually smarter app When you double-press on the Home button, a window raises up from the bottom of the screen, Dock-like, with a list of all the apps you've recently used. When you tap on one of those apps, that app launches--allowing quick switching without going back to the Home screen.