After losing some hair over the issue, and with some help from Marius Kintel, I was finally enlightened as to why my dodecahedron was not coming out properly.

This thing represents some forms of the dodecahedron. In particular, it is combinations of the dual forms (dodecahedron/icosahedron). The .stl files are printed with a radius of 20mm, but you can change that to whatever you want either in the OpenScad, or by scaling.

The 'difference' form is probably the most interesting. It makes for a fairly decent calibration piece. It has some nicely sloping overhangs, bridges, and flat spots. I makes for some good tuning between ABS and PLA as well.

The challenge with the dodecahedron had to do with OpenScad not being happy with the pentagons I was trying to print. They had to be broken down into triangles, which Marius conveniently did for me.