There is alot of questions and worries going around as to whether FSX will support DirectX10 and the implications in regards to Vista support. Mike has already posted a pretty comprehensive article (http://blogs.msdn.com/tdragger/archive/2006/01/23/516653.aspx) which goes over the bulk of the issue. But I wanted to reiterate with a few of the major points.
DX9 and DX10 are both APIs aimed at 3D application (mostly game) development. While Vista will support both DX9 and DX10. The new version of DX10 is targetted only to function on Windows Vista. Windows Vista introduced a new driver model which aims to allow the core GUI of the OS to take advantage of 3D capabilities as well as implementing techniques to mittigate graphical driver crashes (you know, those pesky blue screens).
Since DX10 is Vista only, the need for backward compatibility was elimininated and thus the whole architecture was rebuilt from scratch. Because the basic support required for DX10 is Shader Model 4.0, there is no hardware in existence at the moment which physically supports DX10.
How does this affect FS? Our internal graphics engine is already abstracted from the low-level API used. Which means that adding support for DX10 is simply a matter of writing a specific abstraction layer for the new API.
Do we plan on supporting DX10? Definetly! The question I can't really answer is whether it will be out of the box or as an update to the product. As I have mentioned, there is currently no hardware in existence which supports DX10. And we are still at least a few months away from seeing any type of prototypical hardware (the kind with wires all over the place and tape holding the pieces together).
So in the meantime, we could still do some development using the reference software renderer for DX10. However, at one frame every few seconds, you can imagine how painful this can be. But the reality, is that it would be time wasted which could be more efficiently spent. But as soon as we get hardware, no worries, I will take pleasure into immediately starting work on it! :)