I think that you'll find that the competition is so tight between ATI and NVIDIA neither company will be willing to expend a significant quantity of transistors (hence die space) on large scale features that will not be covered under the DirectX API - from a competitive stand point its far more important (just look at the market over the past 3 or 4 years) to meet the major API requirements, perhaps sprinkle a few fairly insignificant features as well, but concentrate the majority of transistors into improving performance.
As for DX10's release period, I don't think it will significantly effect development as long as there is a fairly good consensus of what is going in beforehand - you might find that a few more features will creep in their developments over time, and they may have to reconsider performance estimations as die space feasibilities may change in that time frame. You might also see that the current generation is pushed out for an extra cycle or so as well.