What causes a Mac to lose its serial number? I upgrade my brand new late 2012 Mac Mini's drive to an SSD and changed the RAM. Now when looking up the serial number in "About this mac" or System Profiler it just says "System serial#" :



The Mini also performed an OS and EFI update before I noticed the problem.
I cannot log in to iMessages and posts an the 'net lead me to believe the non-present serial number may be at fault.

Already tried PRAM reset. Can it be easily reinstated or otherwise fixed  ?



Update:

The only deleterious effect seems to be the inability to log in to Messages with my AppleID. [Earlier edit of this question referenced being unable to download Mountain Lion from the App Store, this issue turned out to be unrelated.] 

Apple Support sent a new machine and replacing RAM and SSD did not cause a repeat of the problem. This machine is able to log in to Messsages.
A lack of a visible serial number should not affect your Mac.

Apple does not guarantee a serial number will be set. Technical Note 1103 warns developers not to rely on a serial number being available to uniquely identify a Mac.

The technical note suggests the serial number can vanish if hardware components are changed. Other reasons appear to be possible but Apple does not enumerate every cause:


  Caveats for using the serial number
  
  It is possible for a system to lose its serial number so that it will no longer appear either in System Profiler or the I/O Registry. Repairing a system by swapping hardware components is one reason this can happen. Apple does not document the specific details of how a machine can lose its serial number. Once the serial number has been lost there is no means to restore it to the machine.
  
  Apple does not guarantee that all future systems will have a software-readable serial number.
  
  Both of these cases illustrate why asset tracking software should always provide a way for a serial number to be entered manually.


I am not aware of any further documentation about restoring the serial number. If it is possible, this capability is likely to only be available to Apple engineers or support staff.