http://arstechnica.com/apple/news/20...pads-brain.ars

Interesting and balanced article on possible benefits of the A4 chip.

Suggests Apples acquisition of the P.A. Semi team in 2008 might have something to do with its efficiency "through pervasive use of power and clock gating. Power gating is a relatively straightforward technique that involves shutting down the parts of a chip that aren't in use. It's harder to implement in practice than it sounds, though, because you have to divide the chip up into blocks that can be put to sleep and awakened independently. You also have to size and arrange those blocks so that the extra delay involved in entering and exiting sleep states doesn't screw up the chip's overall timing.

Because the iPad's LCD is so large and its power draw so great relative to the other components, it's hard to imagine that the A4 gives the iPad more than a few percent battery life advantage vs. a chip like the Snapdragon—in the grand scheme of things for a tablet device, the extra hardware that chips like the Snapdragon and the i.MX515 have on A4 probably doesn't matter a whole lot. But a chip that's really aggressively optimized for the iPhone might give the phone a real battery life and performance advantage over the competition."

But its speculative.