It seems as if the Digg button has a display problem, depending on which browser is being used.
Using Firefox 10, the Digg button does not display. But if I view the same post in Safari 5, it's visible and active.
I have this problem on two sites running WP 3.3.1, and two sites running WP 3.2.1. What makes this even more confusing that the second 3.2.1 site won't display the Digg button in either browser.