It really depends on the manner in which you say it. "(And) who might you be" sounds perfectly polite as long as you don't say it in a condescending tone. "(And) who are you" is a bit more blunt/cold, and I think it sounds rude in many contexts. Of course, Dimcl is correct that another, less harsh way to ask would be to ask "(And) you are...". (Remember to let the end of the question trail off, sounding unfinished.)
There's a moment in Les Enfants du Paradis where Lacenaire talks of the presumptuousness of the question 'who are you?', suggesting as it does that someone can explain their personality in just a few sentences. I think that's why I prefer to ask 'what is your name?'