Blizzard giving away free copies of Wings of Liberty with no discernible pattern
Blizzard has confirmed that this is indeed an experimental approach to promotion, and warned that players who get lucky will simply see the game show up on their account - so don't go hunting for download links or expect to get an email - anything of the like going on outside of battle.net might be a scam.
Blizzard might be trying to get people hooked for the rest of the game by giving select players the first chapter. It might be a sneaky way to get you to log on to battle.net some more, or a way of getting in on some of that sweet free viral promo. Sales might be down, so this could be a salvaging manoeuvre. Who cares, free game!