28nm is two die shrinks from current chip. So 3.1 w/GH * 0.6 * 0.6 = 1.1 w/GH.
You are off by 2x. A shrink from 65 to 28nm theoretically increases power efficiency to 3.1 / ((65/28)^2) = 0.57 W/GH. That's because power efficiency is
inversely proportional to the square of the feature size.
(However leakage is a bigger problem at 28nm than 65nm, which is why 28nm GPUs weren't quite as efficient as AMD/Nvidia had predicted, but combined with whatever logical improvements BFL claims, a 0.6 W/GH number is totally plausible at 28nm. Heck Bitfury does 0.8 W/GH at 55nm!)