The battery saver circuitry overrides the auto dim settings and dims, then shuts off the screen. The only solution is the free 3rd party app, BBLight. You can easily find it by googling.
It allows you to control the backlight. It can turn the light on based on user activity, holster removal, time of day, or based on whether the BlackBerry is running on external power. Try it. It's free.
Posted from my CrackBerry at wapforums.crackberry.com