It means that you think you are really special and deserving of special treatment. It is a way of accusing someone of being arrogant or narcissistic. Depending on the context, it can be said in jest. If so, a proper response would be "Yeah, I'm all that and a bag of chips" :)

I like imk's answer. :) I'll just break down the phrases (they're both idiomatic). "to be all that" = to be something really special "and then some" = and even more (than that).