The Court ruled in Commil USA, Inc. v. Cisco Systems, Inc., No. 13-896, slip op. at 10 (U.S. May 26, 2015), that "a defendant's belief regarding patent validity" does not provide "a defense to a claim of induced infringement".
The holding focused mainly on the fact that the "scienter" element for an inducement theory of infringement relates not to the defendant's belief regarding the patent's validity but to its awareness of infringement. Id. at 9.
The Court also cited the statutory presumption in favor of each patent's validity and noted that "validity is not a defense to infringement, it is a defense to liability." Id. at 10 & 11.
It went on to note that infringers may protect themselves by suing for a declaratory judgment of invalidity, that a good-faith-but-wrong belief in invalidity defense would make patent litigation "more burdensome", and that ignorance or mistake of law generally doesn't excuse breaking the law. Id. at 12-13.
Justice Anthony Kennedy wrote the Court's majority opinion, in which Justices Alito, Ginsburg, Kagan, and Sotomayor fully joined and in which Justice Thomas mostly agreed to. Justice Antonin Scalia authored a dissent that Chief Justice Roberts joined.