When you hit Page Up or Page Down in certain OS X programs (TextEdit for one), the cursor position normally doesn't move; only your scroll position does. As a result, typing or using an arrow key will zoom you back to the cursor position. If you prefer to move the cursor position too, hold Option along with Page Up or Page Down. You can add in Shift to select the text, too.
[robg adds: This won't work in every app; I couldn't make it work in Mail in 10.5, for instance. It worked in TextEdit in both 10.5 and 10.6, though.]
Mac OS X Hints