It's not dimming, it looks dimmer because the sunlight is so bright. Either turn auto-brightness on, use an app like Velis Auto Brightness (which is better than Android's auto brightness function) or turn the brightness up manually when you're in sunlight.
Yup, I noticed my phone doing it as well earlier this year. Extremely irritating and counterintuitive. It was making my phone almost unusable in sunlight. If you turn auto brightness on, it will work correctly, however. I don't really like auto brightness as it's usually too bright for me, but I've got an auto toggle switch for quickly turning auto brightness on and off if I'm going outside.
Mine is on auto and it dims in the sun. How moronic. Makes me want to throw my phone across the room. Last droid did the same thing, so it's not just the Turbo.