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.



From ArsTechnica:

Obviously you need a Thunderbolt cable to connect two Thunderbolt-equipped Macs. I got the 0.5 meter one from Apple, which is quite short, and connected my mid-2011 MacBook Air to a brand new MacBook Pro ("with Retina Display", but that almost goes without saying now). The MacBook Pro has 20 Gbps Thunderbolt 2 while the Air has to make do with the regular 10 Gbps Thunderbolt—but on both the Network Utility reports a link speed of 10 Gbps.

  OS X 10.9 brings fast but choppy Thunderbolt networking