I just purchased an iPad mini (retina) and I've had this repeated experience while reading a book in iBooks of the screen suddenly dimming. If I touch the screen it comes back to the original brightness level and seems to stay there after that. I have the autobrightness feature (in settings) turned off, and I see nothing equivalent in the iBooks brightness controls. I've never seen this sort of behavior in iBooks on my iPad2 or when running iBooks on my iMac or MBA. I don't use other apps enough to know if this is just iBooks, but that's the only app I've had it happen in. It also appears to happen just once per "login." Anyone else seen this behavior? I'm wondering if I need to take the mini back for replacement.