Considering the 13" Pro doesn't have room for a discrete GPU, I doubt we're going to see a retina Air until a few more revs of Intel's integrated GPUs.
One more revision, in fact. Ivy Bridge's 4K display support enabled the 15" macbook pro to exist with discrete GPU switching. Haswell will be faster enough to ditch the discrete GPU.
The iPad needed nearly double the battery to keep up with the larger display+GPU's power requirements. The MBP still had to add 20% to keep up. There's no room in the current Air for extra battery, so even if the current GPU could keep up to Apple standards, running time wouldn't. Which means either becoming less Air-y or waiting for a more efficient part (likely Haswell)
