Cool info on Mac OS X build numbers:
Learn about build versions, such as "Build 10J567", which appear in the About This Mac window and System Profiler (click the word "Version" in the About This Mac window to see the build number).
The build version depends on the version of the Mac OS X operating system that you have installed. When you install a Mac OS X system software update, you get a new build version. The build version of Mac OS X that ships with a Mac may be different than what appears below.