My iPod was recently stolen out of my car, and in the process of reporting it to the police, they asked for the serial number. I was at a loss to provide it, seeing as how my iPod had been stolen.
Anyway, I was playing around in iTunes and went to the iPod tab in Preferences to see if it had the serial number anywhere. It didn't appear to be there, but when I went to click another tab, I accidentally clicked where it said "No iPod Connected." It turned into "Last iPod Connected: XYZ123," where XYZ123 was the serial number of my iPod.
I am using iTunes 6.02 with a 5th generation iPod and the latest firmware, but it may work with older versions.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060120170310468