If your iPod is lost or stolen you'll want to find its serial number. Luckily, your Mac keeps a record of all the iPods that have connected to the computer with the s/n and other info.
The file that stores info about iPods is located at /Library/Preferences/com.apple.iPod.plist. Open it with your favorite text editor.
Here's the hierarchy of the XML file with (descriptions):
<dict> (everything/root) <key>Devices</key> (the section that stores info about the iPods themselves) <dict> <key>XXXXXXXXXXXXXXXX</key> (a unique identifier for each iPod, not useful to us <dict> <key>Connected</key> (the first iPod) (In here is the info about the iPod. Subsequently, I will refer to things at this level of the tree.) </dict> </dict> (ignore stuff in here; it's not useful to us) <dict>
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110413090930559