July 25, 2011
In iOS 4 apps keep running when you open them, and there's an easy way to move among them as you work. Double-click the Home button to produce a scrolling list at the bottom of the screen showing all the running apps, starting on the left with the one most recently used. Scroll through the list as necessary and tap the app you want to switch to. This makes moving back and forth among your mail, calendar, and contacts satisfyingly simple, likewise moving between the book you're reading and changing the music playing through your nearby AirPlay speakers. To remove apps from the scrolling lists, press and hold one till they all start jiggling, then tap the X on ones you want to remove. (You'll need to restart them the next time you want to use them.)
If you scroll left when you first bring up the scrolling list you'll go directly to your music player controls -- play/pause, forward, and back on iPhone and iPod touch (and a control for locking and unlocking the screen in portrait orientation), and on the iPad, play/pause, forward, and back, plus volume, mute, and AirPlay controls.