I think it could crawl its way to 70 million eventually, but won't get much farther than that. I think more realistically it'll end up with around 60-65 million, which still is not bad, considering the first Xbox only made it to 24 million.
But the 360 seems to be in decline already, and I think its best years are behind it, unless Natal is a major success, which I seriously doubt, since most addons don't fare too well, especially expensive ones.