When you install Lion for the first time, you'll probably be excited about the new Full-Screen apps. But if you've read enough reviews or if you're attentive enough, you might think that the dock doesn't show up in Full-Screen apps when you push your mouse down to the bottom of the screen where the dock would usually be.
So here's the secret: You push your mouse down to the bottom of the screen where the dock would usually be. And you keep pushing it down. This way, it ensures that you weren't simply clicking ( or mousing over ) something at the bottom of the screen but makes sure that you were intentionally trying to bring up the Dock.