The new MacBook Air has Thunderbolt 1, thus is still using Displayport 1.1. It takes Thunderbolt 2 to gain Displayport 1.2, with the support for 3840x2160 x 30bpp @ 60Hz (4k) displays.

I'm wondering if Apple is holding up Haswell MacBook Pro updates for a Thunderbolt 2 upgrade. Would make sense for their Pro laptops to gain 4K support ahead of the Air. Just frustrating since I need a Pro laptop for work here soon.