It's the backlight! And it's not hot!
It only warms up if you turn the display brightness to max. I keep it around 50% and its just as cool as my iPad and iPad 2.
Finally someone got it right! The new display uses about 7 watts at peak brightness. The display on the iPad 2 only uses 2.5 watts at peak. This is the
main reason for the bigger battery, and therefore the heat. (Of course the A5X uses more energy too, but not 2.5
times as much like the backlight).
Think, 42.5
Wh / 7
Watts = 6.1
hours (of battery life), definitely a good ballpark value.
If the iPad is set to maximum brightness, it'll only last 5.8 hours on battery without running any apps. If the brightness is set at 50% (the middle) then the iPad happens to last exactly twice as long (11.6 hours when not running any apps and backlight set to say on as well). That data right there tells me where the main energy consumption is, and what produces
most of heat.
The lower left gets warmer than the rest because that's near the warmer part of the logic board, although its hard to say if most of the heat comes from the A5X SoC, but the heat is obviously transmitted by the heat sink which is nothing more than a metal cover. I would assume the relative uniformity of the rest of the iPad getting warm is because of the TWO light bars running along both sides. Again, other than a warmer spot near the logic board, the heat is uniform from the light bars of the backlighting system.
And it's definitely not hot! Every laptop easily get's warmer, I think it's rather impressive that the battery is so big that it can store enough energy to give off a
little bit of noticeable heat for
hours. I remember that it was warmer in the first 30 minutes of first time use, and my first thought was WOW, that's a big battery for a mobile (ARM) device.