New update out.
We'll see if the new thunderbolt update that's out fixes the issue. (Seems to be warming up already 44C at the moment) ambient room temp at the moment is 24C. I've had mine (13" i5) for a little over a week and now the issue is starting. It was fine when the ambient room temp was below 80F (26C), but I live in the Midwest and when the temp outside gets around 68F, the room temp goes into the low 80s.
I've also noticed that the machine retains heat after I close the lid. It looks like it vents through the keyboard possibly? Last night was the first time I'd heard the fan and I've used it for a couple of hundred hours. I shut it down and left it open in front of a fan (so passing air would remove the heat). After restarting I did not have an issue (after about an hour). It had been warm until now, but 80c in the case is too hot. I'm sure it's like anything else, when components get hot, the work less efficiently, which makes them work more, which makes them hotter.
I wondered about this issue before I bought the Air since it seemed too good to be true that a laptop would have no visible ventilation. It is a cool machine, figuratively speaking for the moment.