What's the deal with smashing cake at weddings?

Wedding cake Bettmann

Weddings are an ancient tradition, and over the millenia the various rituals associated with (theoretically) life-long partnership have evolved and changed. One ritual in particular became both prominent and controversial in the West: the act of newly-married couples smashing wedding cake into each other's faces. So where did it come from? Why does it happen, and what do its critics think the practice means?