A large part of that efficiency gain comes from the die shrink between the Cortex-A9 and the Cortex-A15; the A9 was built on a 40nm process, the same as every other mobile CPU sold last year, from the iPhone 4S to the Droid Incredible 2. The shift to a 28nm process in the A15 takes advantage of one of the few “free lunch” aspects of physics – shrinking the transistors means less electricity is lost as heat, so you clock cycles get faster and more efficient, which is win-win for end users. Of course die shrinks are incredibly hard engineering feats, so that “free lunch” is awfully expensive to "cater".
In this case TSMC is doing the catering; this is not a surprise, as TSMC is making just about everyone’s 28nm chips these days. In fact, there’s some concern about whether the yield will be high enough for the volumes that smartphone and tablet OEMs want to ship in the second half of the year.
Speaking of which, the second half of the year is probably the earliest you'll see a Cortex-A15 in a device – with availability starting now, it will most likely be Q4 before we see a real product packing these mobile marvels. Hopefully between now and then SoCs can get an integrated LTE voice chip made on the same die size working, which would finally give us meaningful battery life on LTE phones, even those with normal-sized batteries.
source: ARM via Engadget