Though it's not used in the International Theological Commission's document The Hope of Salvation for Infants Who Die Without Being Baptized, I think the following argument is one of the stronger single arguments against a state of limbo:
Ezekiel and Jeremiah report the Lord's word against what has become a current proverb in Israel, "The fathers have eaten sour grapes, and the children's teeth are set on edge" (Jer 31:29; Ezek 18:2). This saying shall no longer be valid. Particularly in Jeremiah it is clear that the time when "every one shall die for his own sin" pertains to the new covenant which God shall make, when "no longer shall each man teach his neighbor and each his brother, saying, `Know the LORD.' "
This prophecy begins to be fulfilled with the coming of Christ, and is absolutely fulfilled in the eschaton. Therefore, at the end of time, no one will die (be definitively separated from participation in God's life) except on account of their own sin. In God's plan, therefore, original sin is something that is relevant only in this life in time, and does not determine anyone's definitive destination.
This argument does not, of course, attempt to give any account of how salvation in Christ is applied to an infant who dies without baptism, but only argues for the fact of its being offered.