Interesting point. I think this power, while not explicitly stated in the Constitution, falls out of the function of the Court. If there is a conflict between the Constitution and a law under it, the duty of the Court is to resolve in favor of the Constitution. If this power is not inherent in the powers of the Court as defined in the Constitution, then one could only characterize the establishment of such a power by the Court itself as a usurpation.
The Congress passed the Judiciary Act of 1789, which elaborated and specified the judicial powers:
http://usinfo.state.gov/usa/infousa/facts/democrac/8.htm
They actually did a bad job of putting the specific powers of the judiciary into the Constitution. But this act shows clearly that appellate review of questions of constitutionality was one such power, no?
Absolutely, database. Still, I don't think Chief Justice Marshall was a usurper. I think he was trying to "put some flesh on the constitutional bones," i.e., the logic and reason that informed the spare constitutional language of Article III, Section 2 -- to express its meaning in terms of the duty laid on the court by the Constitution. And it doesn't seem to me that he took a real "flyer" in what he did -- his interpretation of the language does not seem the least unreasonable, nor any great logical stretch. I think Marshall's judicial review implicitly follows from the logic of the text.
Marshall was not far removed from the Framers in time, and the Framers' philosophy of government was still vividly current at the time of Marbury v. Madison. In all probability, Marshall shared it.
Plus he doesn't seem to have been particularly "dictatorial" or power-hungry. Case in point: When the prosecution in the Aaron Burr trial wanted to subpoena then President Thomas Jefferson to appear and testify, Marshall issued the warrant. But Jefferson refused to appear, and I gather did not feel he needed to give the court any reason for his non-appearance. Marshall had the good sense to simply let the matter drop -- he did not push it. In this, he indicated a certain respect for the prerogatives of the executive which, in a system of divided powers, I think can be a good thing. I think if he had been a real dictator type, he would have found a way to dragoon TJ into court.
But then, that's only my opinion. FWIW. i could always be wrong. :^)