The two factors behind the curdling of soy milk are acidity and temperature. Black coffee is more acidic than soy milk and can act like a coagulant, making a kind of loose tofu in your coffee cup. Heat accelerates the process; the hotter the soy milk, the less acid is needed to curdle it and the firmer the resulting curds.
We've heard that heating the soy milk first helps avoid coagulation, but hot soy milk will actually curdle more. It seems like the best way to prevent curdling is to warm the soy milk slowly by pouring it into the cup first, then gradually add the coffee. Letting the coffee cool a bit before adding soy milk and avoiding more acidic coffee beans may also help.
Have you ever encountered this problem? How do you keep soy milk from curdling in your coffee?