I enter my Serial number for the online stuff all works. I exit the game, restart and go to the online section and the serial number is still displayed. Makes ‘borrowing’ other people serial number quite easy if you steal from friends; just open their game up.
Since it has to be retyped each time, seems pasting into the entry field doesn’t work, I imagine most people will just leave it entered so they don’t have to bother entering it.
Perhaps this is intended behaviour.