The interesting part is that, unlike all previous A5s (and A4s), which were made on a 45nm process, this one was made on a 32nm process. In general, you'd expect an equivalent 32nm chip to have lower power consumption (and thus heat output) than the same design on a 45nm process.
The even more interesting part... When they looked inside a new iPad 2 (the cheaper one announced at the same time as the new iPad), they found a 32nm A5, with both cores enabled. It'll be interesting to see if the new iPad 2s have better battery life than the old ones; you'd certainly expect it.
And now the borderline conspiracy theory part. The A5X (new iPad SoC) looked at by Chipworks was a 45nm part. But are all A5Xes 45nm parts? Surely, if you have a 32nm process available to you, you'd want to use it for your largest and most expensive, and hottest, chips? Is it possible that some iPad not-3s have 45nm SoCs, and some 32nm?
In a now-notorious test, Consumer Reports found that the new iPad gets hot,