Retina will eventually make it to the iMac, but as others have said, the benefit will be limited and it may take some time for the other hardware to catch up.
There is a reason why Apple first introduced the retina on devices held 1' from your eyes (iPhone), then devices held 2' away (iPad), and then the rMBP (3'). The current 27" display on the iMac is maybe not quite retina, but still extremely high resolution, and I don't see the need or really the benefit of doubling up all the pixels until mid-range kit can drive it without a sweat.
David