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The teardown of the $399 iPad 2 had revealed that it is powered by a tweaked A5 (SoC) chip, which is manufactured using 32-nm process instead of the older A5 chip's 45-nm process.
The 32-nm chip technology make chips more efficient, which in turn is supposed to improve battery life.
Anand Shimpi at AnandTech managed to get his hands on the $399 iPad 2 and has carried out a series of battery tests. He reports that the 32nm A5 (SoC) chip indeed offers significant improvement in battery life.