According to the University of Minnesota Extension, we should let bean pods wither on the vine until they are dry, brown, and rattle-y. I think I picked mine a bit too soon, though they are past their ripe stage and too tough to enjoy.
However, since the next step in seed-saving is to pick them and let them continue to dry indoors, I could be okay. We'll see!
Anyway, I'm in the midst of putting them in a single layer to dry out (and stay dry) until next spring, when I'll break open the pods and try to plant them.