A retina 27" iMac would be incredible. Extremely unlikely, but incredible. I would pay a ton more for it. Just can't see it coming until "4K" displays become the norm, if they ever do.
I mean, realistically speaking, 4K displays make a lot more sense for computers than they do for TV's which you sit ten feet away from anyway.
I wouldn't pay a bean more for it, and to be honest think it's a complete waste of time at best, and a detrimental step at worst.
I can imagine an increase the number of stuck pixels and graphics chips struggling to run things smoothly. Not to mention the increased cost. And all for what? Slightly more tiny pixels?
No thanks.