iPhone starts up dimmed after unlock

Ever since upgrading to 3.1, often when I unlock my phone it comes up dimmed and will not brighten unless I press the power button twice and re-slide the unlock, then it is fine. I've tried holding to a light, etc, to get it to think it should brighten but it seems to be stuck in the dimmed state. This is extremely annoying outdoors. Anyone know of a fix?

iPhone 3G, OS 3.1