I think a private auction house will be something implemented in the first expansion. I am thinking that Blizzard wants to put in guild/clan support in the first expansion and they will tie in new improved AH features with the expansion. One of the main new AH features will likely be some form of private auction houses, such as guild/clan/friend exclusives auctions. It is a good idea and I don't think Blizzard has overlooked this concept, but it seems like a feature that isn't necessary for release. However, it would make a great advertised "feature" for an expansion.