Learned that there's now a 64-bit Cygwin. And that SaVi and Geomview don't work well under it - OpenGL is disabled to prevent a crash, limiting Geomview to software rendering, and there's something odd in Cygwin's piping support that prevents the two applications communicating, unlike 32-bit Cygwin and 32- and 64-bit everything else.
I discovered this while helping out a Geomview user who had naively installed 64-bit using the installer from Cygwin's site, as there was no warning that it was very new and unproven. Still, the instructions for building Geomview under Cygwin have been updated.
Cygwin's an ever-moving target, without stable fixed-point releases that are a known quantity. Exposing users to development may help development, but it doesn't do much for users.