My ds had sleep smiles (and coo's and grunts) from day one and gave us his first real smile right around 3 weeks. By 5 weeks he would smile "on demand" and hasn't stopped since. He is the goofiest, sweetest guy and has the most contagious laugh, which he started working on before his 3 month bday.
That being said, every baby is different, as is shown by the pp's. My sisters babe wouldn't smile (other than sleep smiles) until around 3 months, and is a very happy, smiling toddler.