Illegal vs Unconstitutional: NSA, FISA, and all that
The discussions I have seen so far ignore the important distinction between those two conclusions. The fact that the surveillance violates FISA means that what has so far been done is illegal—and, incidentally, that Bush is a felon, as are lots of people at NSA. But FISA is legislation; Congress can repeal or alter it, and presumably will do so if the appeals court supports the ruling and a majority in both houses believe the current surveillance program is desirable.
Congress cannot so easily repeal or alter the Constitution. So if the surveillance is unconstitutional, that means the surveillance will have to stop.
I should add that, in my view, the illegality of the surveillance has been obvious from the beginning, as I said some time back. FISA was written to control precisely the sort of activity NSA has been engaged in—intercepting communications between suspected terrorists abroad and people in the U.S. It set up procedures for doing so and made it a felony to intercept without following those procedures, or to knowingly use information obtained by such warrantless interceptions.
The only arguments I have seen on the other side are either that Congress repealed FISA without knowing it was doing so when it authorized the use of force or that the President is entitled to break the law in matters of national security. Neither strikes me as plausible. The constitutionality, on the other hand, is a more complicated question, and one that I don't think I have sufficient expertise to answer.
Oddly enough, if Bush believed that what he was doing was unconstitutional that may explain why he did it. FISA provides a two week window after the beginning of a war during which its provisions are suspended—presumably to permit Congress to amend its provisions if necessary. Under the circumstances of the 9/11 attack, it is hard to believe that Congress would have refused to go along with an administration request to make legal the sort of interceptions NSA has been engaged in. But one likely consequence of such an amendment would be a lawsuit claiming that the activity it was authorising was unconstitutional—and if the courts agreed, that would leave Bush unable to engage in surveillance that he, presumably, saw as an important tool for preventing another attack.