I called the hotline and they seemed to think I let the bread rise too long. I went pretty much by the recipe and the bread was looking like it was doing what it was supposed to do but I'll slip it in the oven a little sooner next time and try it.
I decided to bake a loaf of wheat bread (recipe from the king arthur website) using a pain de mie. Everything appeared to be proceeding like it should (rise times, etc) until I took the lid off and the bread had deflated in the center/middle of the pan. I have baked regular loafs of bread before...