Scroll Reverser is a free app for Mac OS X that reverses the direction of scrolling. You can use it with OS X Tiger, Leopard and Snow Leopard to make your scrolling match the ‘natural scrolling’ in Lion.
via Scroll Reverser for Mac OS X.
I use it on Mac OS X Lion in order to use natural scrolling with the trackpad of my MacBook Air and unnatural scrolling with the attached mouse.
(kudos to @romeroabelleira for the hint)