A DX10 demo has made its way online, but it's not from one of the graphcis card companies, nor from any of the benchmark companies or a game developer.

The unlikely source for this demo is a member of the Beyond3D forums called AndyTX and it's an updated version of an earlier demo that he made.

It does have slightly limited DX10 functionallity and the hardware requirements are fairly high.

Not only do you need to be running Windows Vista and have a DX10 enabled graphics card, but you also need a really fast CPU (dual core suggested) and about 2GB of RAM to get the most ouf of this demo.

You can download the demo from here