Click the version number field and it changes to the OS X build number. Click it again, and your machine's serial number is displayed.
But the fun doesn't stop there! You can gain even more information from the login window (accessible when you first start your machine or after logging out). Clicking the field under "Mac OS X" there reveals
- Your computer's name
- The OS version
- Build number
- Serial number
- IP address
- Any available network accounts
- Date and time
We tested this under Mac OS 10.5.2, so if your experience is different, let us know what version of the OS you're using as well as what you found.