Adrian Kingsley-Hughes, writing for ZDNet's Hardware 2.0 blog:

While Apple has undoubted put more power efficient technology into its next-generation iPad — for example, dropping the processor architecture down from 40nm to 28 nm would have resulted in quite a significant power saving — the more dramatic improvement has been the battery itself. Between the release of the iPad 2 last year and the announcement of the new iPad yesterday, Apple has nearly doubled the capacity of the battery, taking it from 25Wh to a massive 42Wh. Measured in milliamps this boosts the battery from 6944 mAh to a monstrous 11,666 mAh. Kingsley-Hughes covers much more in this article that I would like to quote here, but when I was trying to decide exactly what to quote, I found that I wanted to quote entirely too much. Just go read his entire article. Done? Good. I've only seen a couple of sites write anything about this and yet, this is one of the major accomplishments that Apple has made with the 3rd generation iPad. Had Apple not been able to make these battery advances, the new Retina display, 4G networking, faster processor…all together would have greatly reduced battery performance. The tech press would have been howling that the new iPad was doomed to failure. And yet, Apple has miraculously increased the battery by so much, without adding a significant amount of weight or altering the case design, that most seemed to not even notice this great improvement. Most wrote it off as am insignificant accomplishment on Apple's part. A shame, I think.