Hi Steve, also see here for another similar example:

http://moi3d.com/forum/index.php?webtag=MOI&msg=1359.2

So note there that with the base object as a solid, when it is cut it results in the cut up pieces being solids as well - basically the side wall portion of the curve's extrusion gets incorporated into the model in that case.

That doesn't happen if you are cutting a non-solid object, because basically the curve gets punched out into an extruded surface and when your base object is not a solid it doesn't really know how to determine which piece of the extrusion is the outer part (sticking to the outside of the solid) versus which part is inside of it. When you're doing the boolean on a closed solid as the base piece, it's able to then figure out what pieces are outside the volume and what pieces are inside and that makes the side wall pieces come out as well.

- Michael