Seymour (my roomie's new bp) has a yellow belly. His sibling that was with him at the show did as well, but Seymour had cleaner belly lines. I've been reading that it may be a dominant trait and/or a visible sign of being het for something (snow, blizzard, or leucy, I forget which). I think the dominant trait is more likely because he was cbb as a poss pied het and I think the breeder would have discovered by now if he had a whitesnake line running in his collection. He did have a clutch sib who was 95%+ white piebald, though.