The new grease pencil functionality allows you to snap grease pencil lines to surfaces of other objects. Coupled with the surface sketching script, this in effect allows you to draw lines of topology on an existing high-resolution mesh, such as one created in Sculptris, then generate surfaces based on those grease pencil strokes. AFter you have built your cage in this fashion, you can apply a shrinkwrap modifier to snap the cage vertices to the high-res model, and you are done.
I've only played with it for about an hour and a half, now. I have been using a lot of Decimate modifiers lately, simply because I am just too lazy to do the grueling manual retopo that I have been doing. So today I started googling around for alternatives, and happened upon this one. It is odd that I hadn't heard of it. So I downloaded the script and tried to execute it in the Blender script console, but to no avail; Python errors crashed me out. A little bit more googling, and I discovered that as of 2.53, the beta version of the script comes already packaged with Blender as an add-on that must be enabled via the User Preferences/Addons menu. Works like a charm.
The above-linked thread at blenderartists.org links to a couple demonstration videos that show the basic techniques you can employ. I gotta say, this sure takes a lot of the miserable gruntwork out of retopo-ing a mesh.