Having just finished Davis book (2004, original ed.), his contention as I read it is that all types of plaque will have some calcium, both soft and hard. For plaque to be large enough to rupture and cause damage, it would have to have been there awhile (hence calcium present). He points out, rightly I think, that it is unstable plaque that ruptures, and some hardened areas are quite stable. (Many elderly folks have "stable" angina along these lines)
I remember this was a big issue when these scans were first popular: doctors warned people that just because you had a zero score didn't mean you are out of the woods. However, like you, I would be interested what 10 more years of data tells us. I think no matter what, if you are over 40 and have a zero score, you have a very low risk of an heart attack from the traditional cause.