In Panther, clicking twice on the 'Version 10.x.y' text in the About This Mac dialog (in the Apple menu) now gives you the serial number of your machine. In former versions of OS X, this line usually quoted an empty serial number (though in *some* cases it would work, depending on the install method)
I just verified that the number it quotes is actually on a sticker underneath the battery of my iBook, so it's actually getting the serial number from the hardware itself, and not from the install CDs or from a random source. This may come in very handy for admins who need to track hardware serial numbers through an organization.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031111113703708