I've been using iBooks and MacBooks for years, but it never occurred to me until today that you don't necessarily need the screen backlight at all when you are working in sunshine. The display can be fully, beautifully illuminated by direct sunlight. "Frontlighting" instead of backlighting!
The catch is that the screen really does have to be fully illuminated by the sun. Any shade from your own body, or anything else, and the shaded part of the screen will be pretty much invisible. But if you can get all or most of the screen sunlit, you will not need any backlighting at all.
What a great battery saving tactic ... I wish I'd clued in ages ago!
Mac OS X Hints
http://hints.macworld.com/article.php?story=2007091820295886