Romans 1:28-29 "For he is not a Jew who is one outwardly, nor is circumcision that which is outward in the flesh. But he is a Jew who is one inwardly; and circumcision is that which is of the heart, by the Spirit, not by the letter; and his praise is not from men, but from God."

I have to start this post by warning that I'm not very familiar with reformed theology and infant baptism.  From what I understand, infant baptism (such as in the Presbyterian Church), is a sign of the child entering into the covenant community.  It doesn't imply salvation or justification like in Roman Catholicism, but is similar to circumcision in the Old Testament where someone would enter into a convenant relationship with God and His people.

Paul here seems to indicate that the "new" circumcision is not infant baptism, but instead a circumcision of the heart.  It is a spiritual act where the heart his circumcised and it's what is in your heart that qualifies you entrance into God's covenant community.  I'm about to look more into this topic, so if I'm way off, I'll post back :)