iPhone developer chpwn has created a new iPhone tweak called Zephyr that brings multitasking gestures to the iPhone. Zephyr allows you to swipe up from the bottom of any screen to access a multitasking switcher filled with your apps. You can then switch between multiple apps by edge swiping like the iPad allows you to do.
Chpwn says this is the most efficient way to multitask on the iPhone, and that more gestures will be added through future updates. The idea for Zephyr came from a UI concept video created by Max Rudberg. Zephyr is available now through the Cydia jailbreak store for $2.99.