Have you tried covering the sensor near the ear piece...locking the phone...(top button), then while keeping it covered, slide top unlock...? If level is near half way point, it should be dim....then go under a light source of some type....and then uncover the sensor....if it brightens....the feature is working....
You can see this occuring while in the brightness setting page....(level meter will move)
Having a similar problem. I don't use auto-brightness. I have mine at max level (I only change when I need to).
BUT NOW...THANKS TO IOS 6.....
All of the sudden. Whether I'm playing an app, or checking my mail. The brightness level goes completely dim. This has happened with more and more frequency. To the point I can't even see my screen and have to manually turn off the iphone, and then turn it back on.
Very annoying...Very frustrating.
I had the same problem,apparently it works a little differently in ios6, so if your in a dark room turn on auto brightness and set it to your desired brightness for a dark room, and its supposed to remember this setting next time your in a dark room....and when you go out in daylight it will switch back to the brightness you had it set at before you set it for a dark room.. Also once you have set it to the brightness you want for a dark room go outside into daylight and set it to the desired setting for daylight use and I've been told it then detects if your in a dark room and adjusts automaticly and the same if you go into a bright room...I'm trying this,hope it works....