Double-resolution icons were found in "unexpected places" of Mountain Lion by a source who spoke with Ars Technica. Their inclusion was interpreted to suggest Apple could release Retina display MacBooks as soon as this summer.
One double-resolution icon was found in the new Messages application. In the second developer preview of Mountain Lion, released a week ago, some icons are incorrectly displaying at twice their normal size.
Their appearance in the latest build of Mountain Lion led the source to suggest that new MacBooks equipped with Retina displays could appear as soon as this summer, to coincide with the release of OS X 10.8.
Evidence of Retina display Macs cropped up in February when Apple released OS X 10.7.3 with new high-DPI user interface elements. Specifically, a number of cursors in the operating system were updated to scale to larger sizes on higher resolution screens.
Apple added HiDPI modes to OS X Lion last year, but they were previously only accessible by installing Xcode. HiDPI is modeled after the UI resolution doubling that Apple does with its Retina display iPhones, the iPod touch and the new iPad.
Rumors began to crop up late last year that Apple is preparing new versions of its MacBook Pro lineup with double-resolution displays. The resulting display for a 15-inch MacBook Pro would be 2,880 by 1,800 pixels.
Support for higher resolution Macs will come with Intel's next-generation Ivy Bridge processors. Those chips will support up to the 4K resolution, which allows 4,096-by-4,096 pixels per monitor.