I just came across a useful post which summarizes the challenge faced by WordPress webmasters in creating retina display compatible images and then offers a useful WP plugin to handle creating and naming the derivative image for your WP site automatically.
Maybe a similar plugin would be of interest to the Omeka community? Barring that, what's the recommended best practice for adding such derivatives to our Omeka archives by hand?
Given how image-centric Omeka is as a digital exihibit builder, I definitely think being able to deliver retina graphics is high priority. For now as a site developer, I would double all your graphics and add @2x to the end so that you're ready for whatever plugin or functionality is released in the future.
I later found this flowchart which looks to be useful.
Incidentally, I can second the advice there to use http://imageoptim.com/ to optimize the size of your (necessarily much larger) images. Works very well.