That's not quite true as I have seen a few cellphones with properly-working auto brightness.auto brightness has never worked properly in the history of cell phones.
I haven't used AOKP yet myself (although that should change in the next few days), however I do know that similar ROMs have custom auto-brightness thresholds and sensitivities.
Perhaps check your settings to ensure that these values aren't simply different than what you were previously accustomed to?
