We having been waiting with anticipation for the first sign of a social smile from Zachary. We've had a few smiles we thought were 'real' but were then followed quite closely by some milk being brought up, or a bottom burp(!) so were probably wind. L is doing a buzzy bee game with him every day which is getting a good response, but still nothing we'd call a definite smile.
Most of the material we have read (including this), indicates that most babies will start to smile at 4 to 6 weeks, although of course every baby is different and it can take up to 12 weeks for some. Zachary is 5 weeks old this week, so we are hopeful that something may happen soon!
And, judging by the glimpses we've had of what his smile might look like based on his 'wind' smiles, it's going to be absolutely gorgeous.