I was curious from a game design standpoint, is it highly beneficial to develop a game, or even a demo with HLSL being used, or do most of the popular games(like WoW for example) just run off normal hardware techniques? Basically, I know I'm not going to create a popular game or anything like that, however I would like to create some kind of demo or mini game for experience, but would perfer to use the same technologies so I can stay current.

So is designing a new project with dx9 even worth it at this point? Or will dx10 support applications written in dx9?

I personally think, that it will take some time until we see the first DX10 games out in the shelves (...remember DX10 is Vista only). As far as I know atm there is not even hardware available that supports the new DX10 features like geometie shaders. So if you want to get your project going, you will have to use DX9 anyways. However you could benefit from having taken a look at the DX10 documentation before starting off ( DX10 docu comes with the DXSDK). So you are prepared for the future. ;-)

Btw, I have been involed in development of two titels recently and since most companies use middleware, like gamebryo or renderware, I think the "DX10 breakthrough" will not come before those companies made some new major releases that support DX10 rednerers.

GL with your project,
Jochen

And well, since I missed your initial point,
definitly use HLSL, if u go for DX, then also use the Effect framework, since it saves u a enourmous amout of developemnt time.