I'm currently using a Sigma macro/zoom lens with my Nikon D40?

And I'm thinking about upgrading my camera to the new D5100. I know that my Nikkor lens will fit the new camera, but will the Sigma fit, too? You would think that if it fit one and the Nikkor fit both, that it would as well, right?

The lens mount is the same on both cameras.

It should work but check with Sigma, the Sigma lenses are not made under license from Nikon but are, instead, "reverse engineered" most of the time not a problem but Sigma has no crystal ball that will allow their product to be 100% compatible with Nikon's future products.