Just so everyone doesn't get too excited... I have a good feeling the 27" iMac will get a "retina display" as an option and won't replace the existing LCD display at the same price. Much like the MacBook Pro's had a retina model at extra cost at first.
Apple is a creature of habit in regards to new features being introduced.
Yeah, their fuzed glass thing is fairly new, you are right, I don't think they'd just get rid of it and replace everything with Retina's. That said...27" retina is probably a given. But are they going to just leave 21's alone? No retina option? Maybe... but they can't neglect that MP users yet once again, can they?