Marco Arment:

To bring Retina to the 27” iMac and 27” Thunderbolt Display, Apple doesn’t need to wait until 5120×2880 panels are available. They can launch them at the next-lowest common resolution and use software scaling to let people simulate it if they want, or display things slightly larger at perfect native resolution.

This is exactly how I expect Apple to engineer their Retina Cinema Displays.