I imagine that the vast majority of those who have Lion have a Magic Mouse or a Mighty Mouse, but if it is the case that you use a traditional mouse, you will have logically realized that horizontal scrolling cannot be performed naturally.
The trick to get it is really simple: You just have to press the Shift key before scrolling, and then the movement becomes horizontal instead of vertical.
It can be really useful for those of you who have a lifelong mouse, so I hope you take advantage of it.
Source | OSXDaily