As part of its flurry of updates to Apple.com today, Apple's web team posted the new OS X Mavericks background image at 5120x2880. That resolution is identical to the one anticipated for a 27" Retina display for the iMac or a standalone Retina Thunderbolt Display.
With Apple's announcement today that the new Mac Pro would include support for 4K video displays along with Thunderbolt 2, the company is clearly getting ready for larger Retina panels in the future.
Article Link: Apple Posts 27" Retina iMac-Sized OS X Mavericks Background