UPDATE: The Kindle app indeed has a brightness control. It is buried under the font adjustment menu option. Gotta say, points off for crappy UI design on Amazon's part, but I indeed stand corrected. Thanks to everyone who pointed it out.
Apple may have built a great LCD-based reader, but when it comes to favoring its iBooks app over other reading programs, they're playing dirty pool: The in-app brightness control makes a super-dim, extra-comfortable screen—but only in iBooks. The argument against LCD ebook readers generally revolves around backlighting: Too much bombards the eyes and causes strain. I have found that at night, I'm most comfortable reading only with the backlight, leaving my nightstand off. But I only am able to do this in iBooks, because only there can I achieve the super-low brightness setting that makes sense in an otherwise dark room. The Kindle app doesn't have this control, and has to rely on the system setting, which is far brighter. That's why I'm crying foul: If this isn't some kind of deliberate strategic advantage, why not just let the system brightness control go that dim?
It should be abundantly clear from these shots—all taken in sequence in the same location in a dimly lit room using the same manual settings on a Canon T1i—that the iBooks advantage is tremendous. The only question is, will Apple offer its in-app brightness control as an API for developers or will it keep it for itself, in hopes that the subconscious effect on readers will somehow translate into increased book sales? Only Jobs knows.