I would have thought they'll get retina IGZO displays before too long.
Retina IGZO displays may be incorporated for the mid to late 2013 MBA. Whatever the reason is, lack of yields or just holding out, I'm sure it will be soon (speculation). After all IGZO displays were showcased a while ago, and there were 7 inch tablets with that technology integrated. It may be more difficult and more expensive to use IGZO in a 13", but remember that Apple can always add a premium pricetag along with the average consumer priced MBA just like the rMBP line.
----------
There won't be a Retina MacBook Air for the foreseeable future. The issue is having a GPU graphics card powerful enough to drive a Retina display generates far too much heat for a MacBook Air enclosure to dissipate.
This is just my personal opinion. The rMBP 13" has the same GPU as the MBA. With the addition of Haswell leading to a huge improvement in GPU with the Intel 5000 graphics up from the 4000, the MBA can handle it. After all they did put retina in the 13" rMBP.
And as for the heat dissipation, it is true that the 13" rMBP has 2 fans as opposed to the 1 on the MBA. But, the current 2012 barely gets hot for average daily tasks (and after all, the MBA is not for the pros who use intense Photoshop and the like). The MBA remains relatively cool even with 1080P YouTube, movies, word processing, and the like at the same time. With retina it will get hot, but they will allow that just as they did with the iPad 3 where many complained of heat.
And if something were to be a hindrance, it would probably be the GPU and battery rather than heat. Many devices in the past lineup have had a lot of heat production, and not to put heat down as a subject that is to be totally disregarded, but it is at least manageable. If heat is felt, it means the aluminum chassis is doing its job of heat dissipation, and the device just needs to be to the point of being manageable and 'comfortable' though subjective.