I don't have much to add to Rosa's analysis of Sanchez-Llamas and Hamdan, but as an interested party (author of an amicus brief in Sanchez-Llamas) l will elaborate a bit on the general approach to the relationship between international and domestic law that these two decisions represent. First, there are four Justices on the Court that seem skeptical of efforts to incorporate international law into domestic law absent a clear statement of incorporation by Congress. There are four votes on the Court for generous interpretations of legislation to incorporate international law, and perhaps to apply international law in other cases where a statute does not clearly forbid doing so. Justice Kennedy has not made his position clear. In Sanchez-Llamas, all nine Justices described the relationship of the SCOTUS and the ICJ as one of respect but not authority; that is to say, SCOTUS will listen to interpretations of international law adumbrated by the ICJ but feels free to reject those with which they disagree. For five justices, this stance was consistent with rejecting the (in my opinion poorly reasoned) decisions of the ICJ in LaGrand and Avena regarding procedural default. For three justices, this stance led to a (in my opinion heroic) interpretation of LaGrand and Avena that allowed room for a remand to the lower courts to determine whether state procedural rules "gave effect" to treaty obligations without any expectation of a different result. The majority in Sanchez-Llamas also referred to the deference that the judiciary owes to the Executive's interpretation of a treaty to which the US is a party, and the outcome reached was consistent with the Executive's position. The Court did not, however, allow that deference to substitute for its own independent analyis of the treaty in question.
Hamdan, and especially Justice Kennedy's crucial fifth-vote opinion, emphasizes the role of statutory incorporation of the laws of war; the decision does not support the claim that the Geneva Conventions would provide a rule of decision applicable in domestic litigation absent the incorporation that existing statutes mandate, although one can guess that at least four of the Justices would find that outcome congenial.
Lastly, nothing in the opinions suggests any direct limitation on the power of Congress to enact legislation that may be inconsistent with US obligations under international law. Again, four Justices seem strongly inclined to interpret statutory enactments in a manner that would minimize conflicts with international law; four recognize the principle but do not seem to regard it as having a high priority; and Justice Kennedy is enigmatic.
