This early it is just a reflex though you may swear it is a true smile. I think around 2 or 3 months they start to truely smile at you and it is more than just a reflex
This is from babycenter.com:
Now your baby will spend many of his waking hours watching what goes on around him. He'll even flash his first genuine smile, a momentous event for most parents. Soon he'll be an expert at "smile talk," starting an interaction with you by sending a smile your way and gurgling at the same time."