[publius42]

Originally Posted by nobody special

Has the Court ever held that an act of Congress violates the 10th amendment?

Yes, a few times. Wikipedia on the 10th:

As suggested above, the Supreme Court rarely declares laws unconstitutional for violating the Tenth Amendment. In the modern era, the Court has only done so where the federal government compels the states to enforce federal statutes. In 1992, in New York v. United States, 505 U.S. 144 (1992), for only the second time in 55 years, the Supreme Court invalidated a portion of a federal law for violating the Tenth Amendment. The case challenged a portion of the Low-Level Radioactive Waste Policy Amendments Act of 1985. The act provided three incentives for states to comply with statutory obligations to provide for the disposal of low-level radioactive waste. The first two incentives were monetary. The third, which was challenged in the case, obliged states to take title to any waste within their borders that was not disposed of prior to January 1, 1996, and made each state liable for all damages directly related to the waste. The Court, in a 6–3 decision, ruled that the imposition of that obligation on the states violated the Tenth Amendment. Justice O’Connor wrote that the federal government can encourage the states to adopt certain regulations through the spending power (i.e., by attaching conditions to the receipt of federal funds, see South Dakota v. Dole), or through the commerce power (by directly pre-empting state law). However, Congress cannot directly compel states to enforce federal regulations. In 1997, the Court again ruled that a federal act, this time the Brady Handgun Violence Prevention Act, violated the Tenth Amendment (Printz v. United States, 521 U.S. 898 (1997)). The act required state and local law enforcement officials to conduct background checks on persons attempting to purchase handguns. Justice Scalia, writing for the majority, applied New York v. United States to show that the law violated the Tenth Amendment. Since the act “forced participation of the State’s executive in the actual administration of a federal program,” it was unconstitutional.

Originally Posted by Ricky B

So the notion that simply adding a finding to legislation will allow Congress to circumvent the constitution is not something that I accept, and I don't think any member of the court would accept it either.

Actually three did say that was what was going on in the Raich case:

If the Court is right, then Lopez stands for nothing more than a drafting guide: Congress should have described the relevant crime as “transfer or possession of a firearm anywhere in the nation”–thus including commercial and noncommercial activity, and clearly encompassing some activity with assuredly substantial effect on interstate commerce. Had it done so, the majority hints, we would have sustained its authority to regulate possession of firearms in school zones. Furthermore, today’s decision suggests we would readily sustain a congressional decision to attach the regulation of intrastate activity to a pre-existing comprehensive (or even not-so-comprehensive) scheme.