Magic Fingers for jailbroken iPhones allows you to close an applications with a touch of a finger. This kind of App should also be feature on iOS 4.3.
The customizable tweak allows configuration for a set amount of fingers, when touched on the screen simultaneously, to close an app (say 4 to 5 fingers).
You can download the App from the Cydia Store for free.
please comment on your experience with it.