I am actually naked right now, and my MBP15 is on my lap.
But the way I sit puts my knees under the back corners of the machine, and a large part of the bottom middle open to the air. In addition I use fan control, which I have set to keep my fan above 3000rpm and at idle my cpu stays around 45ºf. I have fan control so that the machine stays a little cooler (because heat is the enemy of electronics), but I used it without FC for a year or so and never burnt myself during casual use. I guess I did my gaming and video work at my desk though...