Liza Goitein
Gerard, I think you've correctly described what the Supreme Court would hold. The problem is, when the Supreme Court decided Hamdi, it ignored a key distinction between how the law of war (or "IHL," for "international humanitarian law") treats international armed conflict ("IAC") and how it treats non-international armed conflict ("NIAC"). IHL permits the detention of prisoners of war for the duration of the hostilities in IAC. As Gabor Rona has pointed out, however, IHL neither authorizes nor forbids the detention of fighters in NIAC -- it leaves that issue entirely to domestic law. At the time Hamdi was apprehended, the conflict was an IAC, so his detention was authorized by IHL until that conflict ended. But by the time the Supreme Court issued its ruling three years later, the IAC phase of the conflict had indeed ended (upon the fall of the Taliban), and the conflict was now a NIAC -- meaning that domestic law and not IHL governed the appropriateness of Hamdi's detention. The Supreme Court held that the AUMF (which is, of course, domestic law -- although Rona would argue that the relevant domestic law is Afghanistan's) allowed Hamdi's continuing detention, but it did so by concluding that the AUMF incorporated the principles of IHL -- and those principles don't actually have anything to say about detention in NIAC.
In short, I think the Supreme Court was wrong in holding that the law of war authorized Hamdi's continuing detention in 2004. But I'm pretty sure the Court gets to say what the law is. In light of that (occasionally regrettable) fact, I think Gerard's analysis hits the mark: the AUMF still permits detention post-bin Laden, but may cease to do so if/when we are no longer actively fighting in Afghanistan.
Liza Goitein is Co-Director, Liberty and National Security Program, The Brennan Center for Justice. You can reach her by e-mail at goiteine at exchange.law.nyu.edu