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 ?
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.