My HV has just been round to see my 5wk 3day old baby and asked if he was smiling?
I couldn't 100% say he does! I'm sure I've seen expressions that look like smiles but I've probably passed them off as wind etc so in all honesty I don't really know. I couldn't put my hand on my heart though and say that yes he does smile and it is a regular occurrence. But nor would I say for definite that he doesn't.
She told me that babies should be smiling by now and it's something I need to keep an eye on 
I won't lie, she has left me feeling quite worried and a little bit upset actually.