Ostensibly, the Supreme Court (by a not unexpected 5-3 majority of Stevens, Souter, Breier, Ginsberg and Kennedy, over a dissent of Scalia, Thomas and Alito with Chief Justice Roberts abstaining because of his participation in the lower court's decision) held that the military commissions proposed by the President to try ten Guantanamo detainees (and possibly a number of others) is unlawful, insofar as such commissions are unauthorized by any statute or Congressional authorization, such commissions lack basic procedural safeguards that would be applicable in courts martial (such as the right to even be present and see evidence against the accused), and such commissions violate our treaty obligations pursuant to the third Geneva Convention of 1949.
Talk Left gives us an observation made on Scotus Blog that a key part of the decision appears to be that the third Geneva Convention of 1949 is applicable to the Afghan conflict and apparently to all of the Gitmo detainees, regardless of whether or not the President decided it doesn't.
Readers of this blog, of course, can get significant background on the case from my interview with Salim Hamdan's counsel Neal Katyal, who successfully argued the case to the Supreme Court. (Under the perverse rules of Gitmo, it will doubtless be days, if not longer, before news of the decision can be personally conveyed to Mr. Hamdan by Katyal or his military lawyer Lt. Cmdr. Charles Swift.)
While this appears to be a significant rebuke to the Bush Administration and its purported insistence on making up the law as it goes along, on the "inherent authority" of the "unitary executive", the reality is, the Hamdan decision nominally only effects at most a handful of Gitmo detainees; Rear Admiral Harris, the commandant of the prison facililties has said that, other than halting construction on a second courtroom, he envisions little day to day change at the Guantanamo Bay prison camps after the Hamdan ruling.
And so... life goes on. The irony is that the President was not rebuffed for detaining anyone at Guantanamo, or even for claiming the authority to try these men... it was the insistence on arbitrary rules intended to guarantee conviction (presumably so Bush wouldn't look bad) rather than to use established procedures such as courts martial, or failing that, to have appropriate legislation passed addressing why such procedures should be deviated from. The decision turned on the attempt to deprive detainees of well-established due process rights, not to mention the insistence that the President could put himself above duly enacted treaties such as the applicable provisions of the Geneva Conventions.
Imperial hubris, indeed.