I'm having the same problem as you, Schuby007. I'm running 10.10 on a Thinkpad X61 Tablet. Just as you described, I have the lid action set to "nothing" when on AC power, and when the lid is closed and the external monitor functioning as the only monitor it refuses to sleep properly. If I leave the lid open (not changing any monitor settings) it sleeps just fine. Quite frustrating.