Speaking of OS X 10.9, Iljitsch van Beijnum of Ars Technica:
If you open your network settings in the System Preferences after upgrading to OS X 10.9 “Mavericks”, you’ll be informed that a new “Thunderbolt Bridge” network interface was added to the system. So it’s now possible to network two Macs over Thunderbolt. Let’s take our new network for a spin.
Cool. That said, van Beijnum’s testing showed an inconsistent transfer speed between a MacBook Pro and Air, but only in one direction. Odd.