In Iowa -- If one utilized the late spring nitrate test, they would discover that as a rule if adequate N was applied to a previous corn crop, there would be more carryover N from the corn crop than from a soybean crop.  The bacteria that produce the N on soybeans will do so only when the soil solution around the nodule is deficient in N.  Therefore, as long as the soil solution has adequate N soybeans will not produce any extra and if the plant is not adequately nodulated will be deficient.

The N credit you speak of is true, but the credit is not from carryover N.  There are several factors involved that give a boost to corn following beans (or any other non-grass crop) resulting in increased yields with less added nitrogen.  So the effect is still there, just not for the reason many have previously thought.