My 1st gen and 3G iPhones worked basically the same way for me. I'd set the brightness to about 5%, or very near the left end of the slider, and turned auto adjust on. I almost never had to tweak the settings. I don't like it too bright. With the 3GS, the screen is far too dim with the exact same settings. I have to move the slider almost to 50% to get the same level of brightness. If I turn off auto adjust, the screen becomes blazingly bright at the same spot. But if I turn auto adjust on, the screen is too dim in some situations and too bright in others. As I said, previously, I almost never tweaked the settings...now I'm tweaking them constantly. Could I have a faulty sensor? Anyone else notice a granularity change in the slider?