I've read some stuff before on this, though it was only indirectly related to the subject at hand and didn't go into much detail. I seem to recall that it said that some even add others to the list of sinless. I recall because I disagreed strongly and didn't understand how it was possible. Anyway, I couldn't find that particular passage I was thinking of, but in my search I did run across some stuff about Mary:
Mary was a special case, "for of her we are obliged to grant that her piety had no sin in it." Augustine, too, was obliged to grant this, refusing "out of honor to the Lord" even to raise the question of sin where she was involved; "for from him we know what abundance of grace for overcoming sin in every particular was conferred upon her who had the merit to conceive and bear him who undoubtedly had no sin." (Augustine, On Nature and Grace, 42; cf also paragraphs 37-38 ) - Jaroslav Pelikan, The Christian Tradition: Volume 1, The Emergence of the Catholic Tradition, (The University of Chicago Press, 1971), p.314
Maybe Father Seraphim will be able to better explain? 
I'll look around for some more stuff after work tonight too.