The reason custards crack in the cheesecake is that as teh custard cools, it shrinks a little toward the center. The custard has also laminated itself to the crust, which doesn't shrink. The two forced play against each other. The custard must shrink, and the crust must remain at a constant size. The pressures caused by this differing rate of expansion/contraction is the same as with glass. When a hot glass is placed in cold water, the sufaces touching the cold water shrink more rapidly that the center of the glass, and not being malleable, it shatters. The custard must shrink, but is held fast to the crust. So, the crack appears as the proteins in the custard constrict.
Since I learned how to make creamy fillings vs firm fillings, my custard in the pie no longer cracks. The others who posted before me already nailed it. Don't overcook the custard, it should jiggle, cool slowly, typically in the oven after it's been turned off, and the oven door left slightly opened. Even then, it can sometimes crack. When it does, cover it with sweetened sour cream, or a fruit pie filling. No one will ever know the crack was there.
Seeeeeeeya; Chief Longwind of the North