With more and more players getting in on the all-in-one market, both on the high and the low end of the scale, you just have to figure Apple's going to respond with something new and flash to make everyone go "ooh." There were numeroushints last year about a refresh to the line, and now we have a few more that again point to updated NVIDIA-based machines in 20- and 24-inch glossy sizes. The above photo is supposedly from some secret Apple marketing material, and the company is said to also be reducing stock of the old ones at retailers in preparation of a typically dramatic switcheroo. As far as when that will happen, your guess is as good as ours at this point -- you know how much Cupertino loves surprises.
AppleiMac 27-inch with Retina 5K display (late 2014)