Thanks for the reply, Warlord. I was actually working on something completely different, when I ran across various problems. That test was just to isolate one of the problems. As far as I can see, anything using soft bodied objects seems pretty flaky. In this test everything works as expected when both the box and the ball are dynamic. Changing the ball to be soft bodied results in it penetrating the box and embedding itself inside it.
Adjusting the collision margin doesn't improve things.