Currently Being ModeratedSep 18, 2013 12:10 PM (in response to Fallen2410)
With some Macs that have been Repaired or refurbished that involved the Logic/Motherboard being replaced are sometimes Missing the Serial Number
IN the Apple Menu > About this Mac click twice on the line that tells you the current OS Level.
It will change to the Build Number of the OS and then the Serial Number.
If it is missing it can only be "replaced " at an Apple Store or Apple Authorised Service Provider.
Also see this Apple Document
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously