Two-finger Forward/Back gestures [$35]

It should be possible to go forward and back by using a two-finger horizontal swipe gesture.

This is freaking awesome in Safari on Lion. Check it out: http://www.youtube.com/watch?v=KlBmWgvee-M

Bonus points for 1:1 finger tracking and an animation.

Massive bonus points for doing this in a way that we can easily copy to other apps.