It cant go below the lowest brightness setting unless an app specifically has a function to dim the app rather than the screen.
What may be useful is going to Settings > General > Accessibility and setting Triple-click Home to Invert Colors.
Then pressing the home button three times in rapid succession will invert the screen, which generally makes things a lot dimmer and more comfortable to read in the dark. (when reading a book, I actually prefer to invert the sepia tone rather than black-on-white)
I used to have a new iPad for a couple of weeks and It was so nicer on the eyes, since it was higher resolution and backlight would get so much dimmer.
What if I jail break it? Would it get as dim as the new iPad?
In addition, you may want to combine it with the free and excellent f-lux (dedicated article: http://www.iphonelife.com/blog/87/d...too-blueish-or-just-too-harsh-bedtime-reading )