Professor Orin Kerr has written a careful legal analysis of the president's actions in light of the Fourth Amendment, the Foreign Intelligence Surveillance Act, the Authorization to Use Military Force, and the executive's inherent authority under Article II of the Constitution. His conclusion, in esence:
My answer is pretty tentative, but here it goes: Although it hinges
somewhat on technical details we don't know, it seems that the program was
probably constitutional but probably violated the federal law known as the
Foreign Intelligence Surveillance Act. My answer is extra-cautious for two
reasons. First, there is some wiggle room in FISA, depending on technical
details we don't know of how the surveillance was done. Second, there is at
least a colorable argument -- if, I think in the end, an unpersuasive one --
that the surveillance was authorized by the Authorization to Use Miltary Force
as construed in the Hamdi opinion.
If you are really interested in the rather complex legal aspects of this controversy, rather than the superficial headlines you'll see on CNN, Time, Newsweek, or the L.A. Times, then Professor Kerr's post is an important read.