The aluminum shell of the iPad is an excellent heat sink. There is really no space to move the air inside the iPad so the speakers holes event if they are vent holes would not be at all effective....
Agreed. In fact, blockage of the ports is the least of it.
The "belly effect" is not new. You have two objects - the iPad and your body - that both need to radiate excess heat, and both are being frustrated in their ability to do so. You'd feel similar with just about any laptop or tablet.
Our bodies don't do a very good job of drawing warmth away from the skin; that would raise core temperature, which is usually not a good thing. They're a good bit better at radiating unneeded heat - first, by increased blood flow through the capillaries (convective cooling). When that's not enough, perspiration kicks in to provide evaporative cooling.
The presence of that iPad/laptop blocks the body's convective and evaporative cooling processes. Meantime, the presence of the body blocks the iPad/laptop's convective cooling. Heat from both builds up.
You'll feel this most acutely if your skin is bare and the heat-generating object resting on it is smooth and non-porous. Not only are you without the insulation provide by clothing, but there's nearly no convective airflow, and perspiration can't evaporate. Worse, the film of perspiration provides a heat-conductive seal, something like the silicon "grease" used to maximize heat flow between chips and heat sinks).
