Georgetown law professor Orin Kerr, a former Justice Department official and a regular poster at the right-wing Volokh Conspiracy blog, has a long and somewhat tortured analysis of Spygate, or whatever it is we're calling it.
http://volokh.com/...
The gist of it -- Bush broke the law. He does try to make some odd arguments about how the program might be constitutional based on the power to police borders, but he does say that it violates FISA.
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.