Marty Lederman has commentary on the Hamdan v. Rumsfeld decision today, ruling that the military tribunals set up at Guantanamo are improper, over at SCOTUSblog. Among other things, the Supreme Court has apparently ruled that the Geneva Conventions apply to all detainees captured in the conflict against al-Qaeda. That seems to mean, if Lederman's right, that torture and "coercive" interrogation tactics will no longer be allowed, period. The CIA's interrogation tactics are "officially" illegal, and methods such as waterboarding and inducing hypothermia are now "officially" war crimes. The Court also ruled that the president does not have the power to ignore or violate congressional law.
This looks very significant indeed, and short of convincing Congress to pull out of the Geneva Conventions, perhaps, it certainly looks like the Bush administration has been reined in. What this means in practice, though, still seems very much up in the airpresumably Congress could respond by setting up new tribunals at Guantanmo, or modifying the Uniform Code of Military Justice, or granting the administration other new powers, or so forth
So we'll see what happens.
UPDATE: Glenn Greenwald has a useful discussion here.
UPDATE II: The Court also seems to have rejected one of the administration's legal rationales for its illegal wiretapping program.