From what I've read about DX10, it is supposed to increase performance pretty well over DX9. They are completely starting over and implementing new ways of processing. I think that it's going to be a good thing that it's not backward compatible. This means that game developers don't have to waste time on making it work with a ton of different cards/platforms. They can spend that time making the game better and fully utilizing the DX10 features.
It also means that there will be less overhead for the system because the APIs are specific to DX10
It's also supposed to implement a unified architechture for GPUs. What this will do is, instead of having different parts of the GPU specifcally responsible for an shader operation, like calculations of pixel placement, vertex or gemoetry calcs, they will be able to handle any of those calculations.
What this does is allow for less waiting on one or the other.
Overall It's going to be a good thing, just sucks that everyone has to upgrade to benefit.
I'm looking to buy a laptop. And while it's not going to just be a gaming machine (obviously), I hope to be able to play games on it now and then. I really hope that I will have the ability to upgrade my graphics to something DX10 compatible if I decide to go Vista. If not, it'll kinda suck.