In my mind you are "kind of right" gi7omy........
There are those in the gaming world that have gone out and bought the nVidia 8800 DX10 vid card and loaded up DX10 while still on Windows XP. It works but they are not getting the full benefit. Many are having problems with Vista still.
All the vid card vendors and Microsoft know that not everyone is going to rush out and get Vista, a new DX10 compliant card BUT will buy the DX10 enabled games. To accomodate this, they are developing, shall I say, an enhanced set of DX9c files that incorporate enough "tweaks" to squeeze out some performance and gaming enhancements plus playability for the newer DX10 games that have and will come out into market.
If you are not into gaming, there is no reason to go hog wild on this DX10 compliant cards or even bother upgrading DX9. If DX9 is working fine for you now, I would advise anyone to leave it alone. I am pretty sure they aren't going to work on DX9 anymore OTHER THAN to release an enhanced version for the DX9 compliant card users, like most of us still are, to use with the new DX10 games.
That's my take on it anyway. It is widely discussed in gaming forums I lurk on.