I have heard opposing opinions on this question.
I read a post on another site that said IF you MUST leave it idle for very long, it is BETTER to have it in drive (with parking break set) because things don't get proper lubrication in neutral or park. The guy that made that post is a truck automatic transmission mechanic.
Bottom line is, I don't know what to believe.

 

Idling in drive is just not a safe thing to do from any standpoint when no one is in the truck.
Now I don't know about these funky new automatic transmissions but I can tell you that the old 727 Chrysler transmissions that were used in my 89 and the upgraded version of it that was used in my 95 definately did circulate the transmission fluid in neutral and not in Park. You could put the transmission in Park and the lines would cool off under the hood cause the fluid stopped circulating but a soon as you dropped it in neutral, the lines would heat up almost immediately cause the fluid was circulating again and cooling down. That's why when I drove auto's, I would always shift to neutral at lights and periods of idle time.

 

If your purpose is to warm the tranny. Then place it in neutral while idling. Neutral engages the pump and will then circulate the trans fluid. And yes, if your are getting a significant increase in temp with it in gear, there may well be a problem.