In my Mac Pro review I lamented the state of 4K display support under OS X 10.9.0. In my conclusion I wrote: "4K display compatibility under OS X is still a bit like the wild west at this point". Compatibility was pretty much only guaranteed with the ASUS/Sharp 4K displays if you cared about having a refresh rate higher than 30Hz. Even if you had the right monitor, the only really usable resolution was 3840 x 2160 - which ends up making text and UI elements a bit too small for some users. Absent were the wonderful scaling resolutions that Apple introduced with its MacBook Pro with Retina Display. Well it looks like that won't be the case for long, last night I got reports (thanks Mike!) that the latest developer build of OS X 10.9.3 includes expanded support for 4K displays, 4K/60Hz support for rMBPs and scaled resolutions below 4K.
So, OS X is essentially the only desktop operating system with proper HiDPI support, right?