Hi Raven, yup like Pilou writes above the Construct > Offset > Shell command will create geometry like that.

See here for a comparison of Extrusion versus Shell and the different results that they make when thickening a surface into a solid:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4791.13

Maybe it could be possible for Shell to also be updated in a similar way as what I'm thinking of with Extrude to do an automatic boolean with a solid.

Currently you can get the result that you are showing there by using Edit > Trim to cut the cylinder or sphere with the curve to make a little surface fragment, then use Construct > Offset > Shell to thicken the surface into a solid slab which will produce the "uniform thickness" type result that you are showing there, then delete one face from the shelled result and it can then be joined into the main cylinder or sphere piece to get the final result.

The other surface in a case like that is generated by a "surface offset" of the original surface, and an offset of a cylinder is a larger or smaller radius cylinder, and an offset of a sphere is a larger or smaller radius sphere.

The tools for generating things that use offsets like that are located in the UI under Construct > Offset rather than in Construct > Extrude.

- Michael