It's an identification of the specific version of the OS. All OS X releases have build numbers - run System Profiler and click on "Software" and you'll see a build number for your current version of OS X. Build 10C3067 is NOT an official release but seeing as build 10C2234 was used on the 27" iMacs in November of 2009 it's possible that build 10C3067 is a non-released internal version of the OS build for running the new laptops.
Here's a list of the build numbers of OS X that were included with various systems over the past few years:
http://support.apple.com/kb/HT1159