If you open the About This Mac box, and click "Version", you'll see your build number. If you click again, you'll see your computer's serial number!
Has this been documented anywhere before?
[Editor's note: I've known about this since the Public Beta, but as near as I can tell, it's never been published here before! Someone else might be able to dig it up, but a search on "about" or "build" or "version" didn't find anything.]

