Ambient light sensor readings got quite a bit of an overhaul in iOS 6, and now include Lock screen auto-brightness and better memory management. This has lead to many people experiencing issues with auto-brightness settings. iPhone 5 users in particular seem to be having trouble. If your auto-brightness isn't working under iOS 6, here's how to reset it and get it back in gear.
As long as the brightness slider is moving on its own, you have successfully re-calibrated the sensor and it should work properly from here on out.