As Orin pointed out last December, the strongest argument -- quite possibly an ultimately correct argument -- against the NSA program is that it violates not the Constitution, but the Foreign Intelligence Surveillance Act, which bans "engag[ing] in electronic surveillance under color of law except as authorized by statute." The government has responded that (1) the Authorization for the Use of Military Force implicitly authorizes certain warrantless surveillance, and that (2) the President in any case has inherent constitutional powers, powers that Congress can't limit (and that the Fourth Amendment doesn't limit, for reasons noted below, to engage in national security surveillance). Orin's post (a must-read) suggests that the government's responses are on balance not persuasive, and that the program probably violates FISA.

So it's possible that the court got the result right -- in my view, not on the First and Fourth Arguments, but on the FISA point. Nonetheless, if the court's FISA analysis is mistaken, then the other arguments (the separation of powers and the inherent power arguments) don't provide any independent basis for its decision. The separation of powers point rests on the assertion that "The President, undisputedly, has violated the provisions of FISA"; and the inherent power argument, which is long on rhetoric but short on detailed analysis, rests on the assertion that "Not only FISA, but the Constitution itself has been violated by the [program]." If the First and Fourth Amendment analyses are mistaken (as I think they are), and if the FISA analysis is mistaken, then the other analyses provide no independent foundation for striking down the program.

So, I think (as Orin's post suggested), the real foundation of this decision is FISA. If Congress prohibited this sort of eavesdropping via FISA, and didn't carve out an exception under the AUMF, then the program is indeed illegal (since I don't think the President's inherent power argument much works here, even as to violations of a statute). If FISA doesn't apply, though, then the program is permissible, because there's no First or Fourth Amendment violation here.