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.
Meanwhile, Marty Lederman also has a long post, and observes:
In his press conference this morning, the President focused on two things: (i) defending the legality of his Executive Orders authorizing eavesdropping of conversations involving U.S. persons (including citizens); and (ii) scolding Senators for refusing to reenact the PATRIOT Act.
What virtually no one is pointing out is the incongruity of these two arguments -- that if the President is correct about the legality of his wiretapping protocol, then there is little need to reenact the PATRIOT Act.
ANOTHER UPDATE: Ann Althouse observes: "Members of Congress were briefed about the program in the past and did not see fit to take a position about it one way or the other. They were content to let the President act and but feel pressured to do something now that the program is no longer secret. Let's see what they do."