Several commenters and Scott Lemieux
have convinced me that contrary to what I
said here it wouldn't require a constitutional amendment to impose some kind of supermajority requirement on Supreme Court decisions.
UPDATE: Of course at the end of the day this would need to be litigated. In principle, the Court could vote 5-4 to strike the law down which would create awesome legal paradoxes.
This article available online at:
http://www.theatlantic.com/politics/archive/2008/07/changing-the-rules/49035/