If you permanently disable this feature, though, you'll lose its battery-saving benefits even when it makes sense--like when you walk away from your machine for a minute or two. A nicer solution would be a way to temporarily disable it when you don't want it active. It turns out you can do just that with the Desktop & Screen Saver System Preferences panel.

Open that panel, click on the Screen Saver tab, then click on the Hot Corners button. In the new sheet that drops down, decide which screen corner you'd like to use (I use the upper right corner), then click the menu button next to that corner. On the menu that appears, select Disable Screen Saver and click OK.

From now on, when you start reading a long document--or do anything else where you'd rather not have the screen go dim--just flick the mouse into your "do not activate" corner, and the screen will no longer dim. (And, it should go without saying, the screen saver will not come on.) After you're done reading, move the mouse out of the hot corner, and the screen dimming function will again be active.