In technology, speed is almost universally the enemy of power, and that rule certainly holds true in the world of cellular data standards. You heard it from His Steveness himself -- one of the main reasons we don't have a 3G iPhone on store shelves is because he isn't happy with battery life on HSDPA chipsets yet
(consumers' opinions be damned, apparently), and in general, runtimes on 3G handsets large and small flag their 2G cousins, sometimes by a significant margin. Happily, the wiz kids at NTT DoCoMo are on the case, crafting Super 3G / LTE chipsets on 65nm dies capable of burning through 200Mbps at "sufficiently low power consumption" for mobile use. There's still no word on when NTT DoCoMo (or anyone else, for that matter) will be launching a commercial network at LTE speeds, but the company's hailing this latest batch of chips as a "milestone" on that journey -- and let's be honest, odds are good that Japan's gonna be rocking this stuff years before the rest of us anyhow.