   Dynamic addition of verticies to model
  I see (in the docs) one can use the tessellator to add vertices to a mesh does this use CPU or GPU to do the interpolation?How does one use the GPU to add extra vertices to a model. For example if you were drawing a series of lines how do you get the GPU to interpolate 3 input vertices to 30 output vertices to go into the VS? (It seems a waste to do the interpolation on the CPU side in a dynamic VB and then pass all that data over to the GPU). Can it be done in HLSL or using the DirectX interface to the tessellator?   
  No, you cannot use custom tesselators like that.  The API has support for some stuff like that (ie n-patch tesselators), but hardware doesn't support it very well (ie there is no support at all, or its wicked slow).  The response to this has been the development of many techniques such as normal mapping and parallax mapping that add details to models without actually adding vertices.  However, it doesn't sound like this is what you are really going for.Theoretically, when DX10 rolls around, you could use geometry shaders for this.  For tesselating a line, like you mentioned, it might be able to handle it.  However, don't count on the performance being there for any type of realtime or dynamic tesselation of any magnitude.   ;  Thanks circlesoft, if the tesselator is going to be slow and wont run on most cards then I will do it another way. Expect there is some fast way of doing this using the instancing API but again the hardware support is lacking. Will have to ask Mr CPU if he has time to do this :-)   