I honestly didn't realize that the Hamdan decision was set to be released today (to be completely honest, I didn't know the case was named Hamdan, although I did know there was a case being argued about the military tribunals). I started reading current Supreme Court cases with Hamdi and Padilla, so I can't complain about Hamdan (PDF).

Roberts sat out Hamdan because he already had a shot at the case when he was an appellate judge. Since Roberts' decision ruled for the President's military commissions, a tie vote would have left that in place (without being binding on other apellate courts, but a law passed last year requires all challenges to the commissions to be filed in Roberts' old courtroom). The Court ruled 5-3 against the commissions as they currently exist. One reason I like to read the actual opinion is that there's going to be a lot of spinning about how this is a setback to the President, or how bringing in international law (in this case the Geneva Conventions) is a terrible mistake, or how this is actually a victory for the commissions in general.

I haven't actually finished the Court's summary yet, but important parts that stand out include the statement:

The military commission at issue is not expressly authorized by any congressional Act. Quirin held that Congress had, through Article of War 15, sanctioned the use of military commissions to try offenders or offenses against the law of war. ... Quirin recognized that Congress had simply preserved what power, under the Constitution and the common law of war, the President already had to convene military commissions.

Since the US signed the Geneva Conventions, they are a very clear part of the current laws of war. In fact, it seems pretty clear that the "common law of war" is basically international law. So, yes, this is a decision that's going to include international law.

The military commission at issue lacks the power to proceed because its structure and procedures violate both the UCMJ and the four Geneva Conventions signed in 1949.

The rest of the decision maps out where the commissions (designed before the UCMJ was passed) violate the UCMJ. So, the Court did not strike down the idea of using commisions, but it did decide that these commissions need overhauled before they're used. I'm sure Congress will be writing that overhaul next week.