Ya, the torque converter (basically 2 fans suspended in trans fluid, one connected to the engine, the other to the trans) is what allows you to stay in gear even with the engine idling. But it takes more power to do so. So I can totally see why you would increase mileage by puting it in 'N' when at a light.
This is really an awesome thread. It is showing very simple things to do to increase fuel economy. These are things I would have never thought of as I havn't owned an automatic car in a while.
I wish there was a cheaper obd2 data tool out there. I think everyone that can afford it should have a scangauge. Just watching your MPG rise by doing simple things like putting your car in N must be amazing.