After having used my new 12" PB all day, I will say that it can get rather hot if you don't have it on the right surface. The biggest contributor to overheating seems to be obstruction of the vent under the hinge.
When I use the 'Book in my lap so that the hinge vent is in the open space between my legs, the machine gets rather warm but I wouldn't say it gets hot. In fact, the fan hasn't come on all day when it's been in my lap.
However, this evening when I had a cat in my lap, I set the 'Book on the armrest of my recliner. It got a lot hotter there; the fan actually came on eventually (it is a very quiet fan, BTW).