Would it not make sense that, since the book is a fictional story, the mysterious sixth family represents the REAL New York mafia? I know that you can draw similarities between the others and real life families, but since the sixth is hinted at but never described (nor is their part in the war mentioned in depth) I always thought it was supposed to represent a real "Family" of New York.

I subscribe to the "five families INCLUDING the Corleones" theory, but I still find it odd that the meeting explicitly says that there were five OTHERS besides Don Corleone. Until I read that part I just assumed that "The Five Families" was a generic term for all of them. I'm not so sure now, though...