I learn from Wikipaedia that 'so' is what linguisticians call a 'discourse marker', that is, a word or phrase that is relatively syntax-independent and does not change the truth conditional meaning of the sentence. Other examples are 'oh', 'well', 'now', 'then', 'you know', 'I mean', 'frankly', and 'as a matter of fact.' It has been suggested that a discourse marker shows the connection between what is being said and the wider context. It either connects a sentence to what comes before or after, or indicates a speaker's attitude to what he is saying.
Here are some more examples of what seem to me to be quite appropriate uses of 'so' at the beginning of a sentence:
'So [it follows from what I’ve just demonstrated] the butler did it’ .
‘So [now we’ve both got our drinks] tell me about yourself.’
‘So [having got that out of the way] let’s move on.’
‘So [to change the subject to something more interesting] who’s that very attractive young woman?’
‘So [given that you were only just now being so nice to me] why are you threatening me with that meat cleaver?’
‘So [as you’re such a keen student of the turf] what’s going to win the Derby?’
‘So [let’s get to the point] when did you last see your father?’
‘So [to set the scene for this story which I’ve decided to tell you], this bloke walks into a pub....’
‘So [are you trying to tell me that] is this all there is?’