There's a good possibility we shall see a retina iMac but I would wait at this point for haswell regardless if it has retina or not. A lot of members here think it will come late in this year.
It will be interesting to see what Apple determines "retina" to be on an iMac. For the 27", I really, really doubt we'll see a 5120x2880 resolution. It's rather unnecessary due to the distance you view the iMac display at.
That said, I certainly wouldn't be averse to a slightly higher resolution iMac. No doubt about that.
Scrap that, - let's see 5120x2880, Apple! And saying that, with the huge amount of empty space inside the 27" iMac, I'd LOVE to see Apple power that ridiculous resolution with a Geforce 680MX GTX SLI setup. That would be fantastic, and really set the iMac into whole new levels of power. There's certainly room for a larger power supply and dual video cards, though the logic board would need a huge redesign.
So what, though? DO IT, APPLE!