I had the logic board replaced multiple times on my powerbook, each time but once it came back without the serial number. The one time it came back with the number I check the 'Recent Items' list and sure enough an app was listed with the name to the effect of 'Hardware Serial Restore'.
I did a litte searching online and found out that it is not impossible to restore the number, but it takes a program that not all Applecare Centers or techs have access to, or feel the need to use.