Apple has revealed that content will move in the same direction
as your finger when scrolling inside of Lion via your mouse or
Magic Trackpad. You scroll down, you move up in your document.
This is exactly the opposite of how it's worked ever since the
scroll-wheel mouse hit the market 16 years ago, Debates on how
good or bad of an idea it is aside, here's how to bring that new
functionality to Snow Leopard before getting your mitts on Lion.
Just install a free utility called Scroll Reverser. It adds an
option in your menu to modify how your scroll wheel functions.
Do you want content to scroll in the opposite direction  or same
direction as your finger? Then activate and deactivate Scroll
Reverser.  That's it.  [Via Macworld]