Apple changed the way scrolling works in Mountain Lion, presumably to make the motion consistent with the way it works on iOS touch screens. They call this "natural" scrolling, and it's enabled by default. To disable it, go to
System Preferences ->
Mouse and uncheck
Scroll direction: natural.
FWIW, it took me only a few days to become used to the new way; it's very intuitive, once you've unlearned a couple decades' worth of habit.
