I've got a Nexus S. There appears to be some kind of setting related to power conservation which makes the string automatically go black to conserve energy. It is really really really annoying for two reasons: 1) it starts to happen when my batter is still something like half full and 2) it will go black when I'm right in the middle of doing something -- it seems to completely ignore whether I am using the phone or not.

I've tried finding any settings related to power conservation and can't seem to sort this out. Help!