ProSwitcher. This cydia (jailbreak) app allows you to switch between active applications using palm pre like interface of cards. It will enable and install backgrounder for you so that you can multitask on the iPhone. Works well on iPhone 3GS only as it is the only phone with enough speed and memory for effective multitasking.
As a side note, i have also found a cydia app that will disable the voice control of the phone which i never use and has the tendency to activate on its own. With this disabled, i can use the Press Home and Hold activation method for ProSwitcher without triggering voice control.
Just love my iPhone