Code contained in the OS X Yosemite
beta appears to point toward support for future Retina iMacs. A forum poster at MacBidouille
notes that in a file covering graphics scaling settings, the highest resolution available is 6400x3600; for a Retina panel, that figure would effectively scale down to 3200x1800. Other resolutions mentioned
are 5760x3240 (2880x1620 in Retina), 4096x2304 (2048x1152), and still lower settings.
Retina displays have been present on iOS devices and the MacBook Pro for several years at this point, but Apple has yet to bring the technology to the iMac, the only Mac desktop with a built-in monitor. The main obstacles have presumably been the difficulty of manufacturing a Retina display in iMac sizes -- 21.5 and 27 inches -- and keeping costs down so that the computer doesn't become substantially more expensive. Apple could make the switch this fall alongside the release of Yosemite, since it has yet to ship a 2014 iMac, but that's uncertain.