No Serial Number on Mac

So if you click on about this mac, then proceed to more info, you're telling me that your serial number doesn't show up?

You are saying the area shown by the pink arrow is blank?

http://support.apple.com/kb/HT1941?viewlocale=en_US&locale=en_US

The article from b j t is correct, and a way to locate the serial number.

However, I've also had an issue with a MacBook Air when the serial number did not show up on the "About this Mac" display as SupaDynaMike shows above. The issue arose when I had to take my Air to the Apple Store to have a motherboard replaced. They were able to reinstate the serial number so it could be seen from software when I pointed out the issue at a Genius Bar.