Senator Wyden says, " It may be possible for the U.S. to implement ACTA or any other trade agreement, once validly entered, without legislation if the agreement requires no change in U.S. law..." but "...the executive branch lacks constitutional authority to enter a binding international agreement covering issues delegated by the Constitution to Congress' authority". However, then he states that "...if you allow the USTR to express your assent to ACTA, then the agreement can bind the U.S. under international law even without Congress' consent, because international law, not U.S. law, determines the binding effect of international agreements. According to many international law scholars, customary international law recognizes the ability of the chief executive of a country to bind its nation to an international agreement regardless of domestic legal requirements."
So while the treaty won't stand up before judicial review inside of the United States, it can still be considered binding in International Law. You then have to determine which has greater sovereignty in American courts.