I just can't see retina screens on the iMac anytime soon. Maybe Apple will launch a seperate smaller sized retina iMac model a year from now, but as far as 27'' retina, that's definately not gonna happen for some time. The only way would be to drastically reduce the "stock" resolution, if they're gonna use the double resolution-scheme (or quadruple if you will).
Personally I'm buying the new iMac with no reservations, for me the 27'' screen is a must (I vastly prefer that to the 21'), and it's gonna be at least 2 years before we see a retina 27'', if not more.
Not that have a crystal ball into the future or anything, but from looking at projected increases in screen resolutions at least 2 years is a pretty safe bet.
The display tech is there imho, maybe 6 months or so away, hell ibm pulled it off 10 years ago, igzo is there from sharp to allow tighter placement of transistors, 4k or other hidpi monitors pop up now and then... BUT...
Apple has two problems at hand:
A. The software implementation of the double rendering then halving to fit the hidpi monitor, means huge stress of the gfx, they are not only pushing millions of more pixels, they are doing double that, and then rendering at a lower resolution...hence the lags and stutters of the retina mbp at the moment in simple ui animations and motions. It's a software hack, a smart one, but one that leaves apple software not doing a lot of work (in typical recent fashion) and offloading it on increasingly superb hardware. But the hardware isn't there yet as you said, and they have to wait,, all the more so, since in their infinite wisdom they are slimming the smaller imac and adding portable instead of desktop components...
B. Samsung. They shouldn't have messed with them. AU optronics is major corp, and one of the very few major lcd players, but they are not samsung. Sharp has the screen tech but they dont' have the manufacturing to where it should be and they are haemmoraging cash all the time. Apple also have a tv set in the pipeline to put more strain on the supply demand of things. Can lg and au step up AND offer decent yields for retinas?
These factors are pushing things further back, and to be honest, I will be very happy (what with their constant recent touting and self patting on the back) stick it to them and come up with a "retina" stand alone panel first. That would be fun, the new imac with a retina display, that's already out by another manufacturer... I wouldn't put it past them to spin it some way to their favour...
Apple's shameless marketing, personified by Schiller (now a loose canon without Steve Job's tutelage), makes them the only company in the world that can claim, with a straight face, that they are coming out with a longer phone because all the other bigger screen 4-5" phones can't be held with one hand, and then proceed to claim they chose an 8" diagonal screen for the ipad mini, so that it could be held with one hand, and even go as far as to show a hand (one from Gulliver's travels I suppose) actually holding the device!!!! So utterly absurd it's priceless!!!