Why shouldn't you?
It's annoying right? Why should you have to relearn how scrolling works?
Because it makes no sense in Lion, and I'll bet you anything it'll make less and less sense going forward. This is the new paradigm, learn it now or later.
In the beginning of Graphic User Interfaces scrolling was done by clicking the scrollbars on the side of an application window.
Since this wasn't a very efficient way to do it many weird solutions for simpler scrolling popped up here and there. It soon became standard for Mice to have scroll wheels on them. Making the entire representation of scroll bars a bit redundant. They take up a lot of screen real estate just to show you where in a window you are looking at any one time. It's not like you didn't scroll there in the first place right?
When touch pads started becoming standard, this design thought was transplanted over from mice and scroll bars. Nothing wrong with that, reinventing the wheel isn't always a good thing.
Except when it is.
In this case it made no sense. The mouse and it's scroll wheel use two different controls to achieve two different things. You move the mouse to point. And you scroll the wheel to.. eh.. scroll.
But on a touch pad you use the same control. Your poking the touchpad to move the pointer and then poking the touchpad in the opposite direction to scroll. The only reason this feels "natural" is because we, as the ingrained PC users we are, are so used to scrollbars. We know that what we're scrolling isn't the content but the scrollbar. Which in turn scrolls the content...
See where the design falls apart?
The metaphor is broken. The scrollbar no longer makes sense when you scroll using the pointing device to move the content, instead of the scrollbar.
Alright. That makes sense, but why relearn? Why fix what ain't broken?
In two words: Cognitive load.
Lion's natural scrolling (directly scrolling the content instead of the scroll bar) will become the standard, like it or not, because the average PC user doesn't change default settings and certainly don't understand why scrolling should be inverse to the screen. The cognitive load of thinking about how to scroll will simply become to much as more computers are delivered with touch pads and more of our PCs become touch based (as tablets become more widely spread).
To clarify; on a mouse the scrolling direction won't change. Because the scroll wheel isn't directly linked to the content anyway. But a touch pad is directly linked.
Update: For some reason, Apple has changed the scrolling direction on the mouse wheel for non-apple mice. This is weird. Thanks to Dan in the comments for reporting!
It takes a little time to get used to, though less than you might think, but it will be worth it. And you won't have to relearn later on which will get increasingly frustrating.
Not convinced? Check out MG Siegler's excellent pre-lion post The iPad Has Broken My Brain; OS X Lion Will Help Fix It.