I thought I was crazy yesterday because my screen never would turn off. it would only dim and then I would have to amnually hit the power button to get it black. I rebooted the phone and no problems. Didn't know what it was or even that it was related to the HDMI cable until I read this post from SHAMUS:
I'm noticing a weird trend with my battery life.
There is a bug where once connected/disconnected from an HDMI cable or dock, the screen loses it's timeout feature. It dims but does not time out. You have to manually click the power button to turn the screen off. Sunday I forgot about this bug and the screen ended up staying on (dimmed) for many hours but my battery life seemed better than the previous day. I tried it again yesterday with similar results. Nothing else had changed.
I'm going to try a more scientific approach to this today and see how it goes.
So I am not the only one. anyone else have this issue??? I used the mirror mode when I plugged into my Flat screen.