I really wouldn't use the Federalist Papers to justify Constitutional intent, or even Madison's true opinion on any subject he wrote within. He wrote them because though he thought the Constitution as written would fail, it was the best they could do at the time and would hold the Union together until it could be changed, which it was.
More importantly, Judicial Review was created and filled the gap Madison saw between the powers enumerated in the Constitution and their implementation.
Within the mainstream of jurisprudence, as well as good case law, the Federalist Papers are absolutely used to clarify and explicate the original meaning of the Constitution. They are far and away the leading source for that purpose.
If you're interested in how well the Papers do (or don't) reflect the intent of the framers and contemporary understanding of the Constitution, I can suggest a 40-ish page law review article [0] by Gregory Maggs. It also covers the history of the Papers' citation in legal scholarship on constitutional interpretation and in decisions of the Supreme Court.
That's really interesting. I'm not a lawyer, just someone who did his thesis on Madison, specifically why the Constitution didn't fail in the way he thought it would; I can only speak to that. I'll read the PDF, thanks.
If you want, the Library of Congress has copies of his correspondence and notes, they're a really interesting read. You can also find them at http://oll.libertyfund.org/index.php
Pash is right, but I think he overstates his case a bit. It's important to realize that there are as many original interpretations of the Constitution as there are framers, and that the Federalist Papers represents the views of a certain subset of the people who wrote the Constitution. They are a popular reference, but they are by no means gospel.
More importantly, Judicial Review was created and filled the gap Madison saw between the powers enumerated in the Constitution and their implementation.