From the SCOTUSBlog:

The Supreme Court ruled on Thursday that Congress did not take away the Court's authority to rule on the military commissions' validity, and then went ahead to rule that President Bush did not have authority to set up the tribunals at Guantanamo Bay, Cuba, and found the commissions illegal under both military justice law and the Geneva Convention. In addition, the Court concluded that the commissions were not authorized when Congress enacted the post-9/1l resolution authorizing a response to the terrorist attacks, and were not authorized by last year's Detainee Treatment Act. The vote against the commissions and on the Court's jurisdiction was 5-3, with the Chief Justice not taking part.

The Court expressly declared that it was not questioning the government's power to hold Salim Ahmed Hamdan "for the duration of active hostilities" to prevent harm to innocent civilians. But, it said, "in undertaking to try Hamdan and subject him to criminal punishment, the Executive is bound to comply with the Rule of Law that prevails in this jurisdiction."