You can obtain the Mac OS X Server serial number (for Tiger) via the command line. At the Terminal on the server itself (or via ssh if you wish), type:
more /etc/systemserialnumbers/xsvr
Press Return, and you should see the serial number displayed. I can't help but think that this could be made more secure rather than being stored as clear text.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060710091805963