The number one requested iPhone feature multitasking is tackled in this "What if iPhone had..." concept by Ocean Observations. There are several benefits of multitasking, and we believe it will come to the iPhone sooner or later.
Our idea is to present the running apps together with the rest of your app icons on the home screen, instead of hiding them behind long pressing some unrelated key. All running apps have a miniature representation in a cover flow at the bottom of the screen. To make room for this, each home screen page will have one less row of icons and the dock is removed completely. But since the running apps often will equal your favourites, they will still be easily accessible.
You can tap or swipe any of the cover flow miniatures to move it into focus. Then tap it again to bring it to full screen. Once an app is full screen, double-tapping the home button brings up the cover flow, letting you jump between running apps. This extra feature could be the new default setting for double-tap, which you could still change in the settings. Tapping the home button once minimizes the app into the running apps cover flow. The apps are arranged by the order you launched them, with the most recently launched app to the left.
A running app is also indicated by a backlighted app icon with a blue text label on the home screen. It's still possible to bring a running app to full screen by pressing its icon.
To close an app, drag it out of the cover flow and the miniature moves back to the app icon on the home screen. The app icon then changes back to its original state. Depending on the performance of the phone, there could be a maximum number of running apps. In that case, when launching a new app, the least recently used one could be closed automatically.