The interface for multitasking is slick and convenient, especially for one-handed phone operation. Double-press the Home button to activate the multitasking bar; the rest of the screen subtly moves up and becomes hidden behind a translucent screen. Then, simply flick horizontally to find the app you want, and tap its icon to return to that app. The apps appear in the multitasking bar in the order you last used them; for example, the most recently-used app moves to the front of the line at left, bumping everything else to the right.

Apps written to take advantage of multitasking have two states: running and suspended. iOS 4 tries to keep as many eligible apps running as it can hold in memory at one time; the other apps remain suspended (until purged by iOS 4 or closed by the user). If the app is written to take advantage of this new feature, it will pick up precisely where you left off.

iOS 4's multitasking interface certainly works smoothly, just as it did when I first at Apple's Worldwide Developers Conference. But at WWDC, I was limited mostly to bundled Apple apps, not the bevy of apps I know and love from my own iPhone. Now, on iOS 4 launch day, it turns out that Apple's own apps, again, are currently the best candidates for multitasking. As of this writing, relatively few apps , and of those, not all are optimized to take advantage of the multitasking.

A that, as of this writing, about 225 apps are "iOS 4 tested," a pittance considering the more than 200,000 apps in Apple's App Store (but not surprising given that Apple only began approving iOS 4 apps last week). But when I changed that same Google search to include the term "multitasking", and only today, including Pandora Radio (which indeed continued to play as I used other apps, and which resumed where I left off after I used a game app that needed audio), ESPN ScoreCenter (which continued seamlessly in the background), and How to Cook Everything (popped me back where I left off, without having to re-open the app).