Yesterday I set about generating 3D noise, in particular, texture maps for 3D planets. It sounds like a relatively straightforward extension of 2D noise, but unfortunately it didn't turn out that way.
First up, a SphereMapper generator. This handy little class takes a 2D coordinate, and transforms it into a 3D coordinate on the surface of a sphere, just basic Polar to Cartesian conversion. Of course, this generates 3D coordinates on a unit sphere, and while the front looked all right, the back looked absolutely terrible.
Turns out my Voronoi implementation didn't work correctly with negative coordinates. This required only a simple fix, but unfortunately that fix required axing the optional tiling qualities I had added in the day before.
Then it also turned out that my Simplex implementation didn't work with negative coordinates either. I haven't figured out a fix for this yet, so in the meantime, I have implemented a version of Value Noise (as described by Hugo Elias). This is a fairly decent approximation of Perlin/Simplex noise, and does work with negative coordinates, but the quality is a little worse, and the high quality version is considerably more expensive than simplex noise.
I will have to fix the simplex noise at some point, but in the meantime, value noise is a good generator to have, and it makes for pretty decent looking planets.
I also notice that my OpenGL sphere class has a lot of texture distortion in the polar regions - far more than should be expected. Apparently generating a sphere out of stacks and slices isn't as straight forward as one would imagine...