Adafruit is now selling the “Qualia” 9.7” DisplayPort monitor, with 2048x1536 resolution.
It’s essentially a retina iPad LCD, with a small circuit board attached. The circuit board accepts a full-size DisplayPort connector for video input, and has a connector for the provided wall wart power source. Adafruit provides a 10’ mini-DisplayPort to DisplayPort cable.
While it’s been possible to use a retina iPad as an external display, it required connecting via wifi, using the AirDisplay app. This introduced quite a bit of lag. The DisplayPort connection on Adafruit’s monitor provides a full-speed wired connection, so it’s as fast as any desktop monitor.
Adafruit provides a laser-cut plastic stand which you have to assemble, but it’s easy. It only has one usable angle, which is a bit too upright for my taste. I’ve suggested that they provide something with holes drilled for a VESA mount.
Editing to add: This could be useful for a Mac or iOS developer, who wants a way to check that their app looks okay on a retina display, without buying a retina MacBook Pro, or having to install the app on a retina iOS device. It may also be useful for graphics apps.