Mac OS X: About This Mac window provides computer serial number
You
can use the About This Mac window to find your
computer's serial number when started up from Mac OS
X 10.1 or later.
Follow these
steps:
Choose About This Mac from the Apple Menu.
Double-click the version text,
for example "Version 10.1".
Your computer's serial number now
appears where the version text had been.
Note: Some earlier Mac OS X-compatible computers do
not have their serial numbers stored in a way that
allows this feature to work.
Lack of this feature on your computer does not indicate any software or hardware failure.