Over on Reddit it has been noted that Blizzard is giving away full versions of Starcraft II to a bunch of random people, for no apparent reason.
One possible explanation is that Blizzard is going to make the first installment, called Wings of Liberty, free to everybody. At this time there is no official press release or any sort of notification from Blizzard themselves. Hopefully they will clear things up soon.
Did you by any chance receive a free copy of Starcraft II?