according to this source:

President had legal authority to OK taps

President Bush's post- Sept. 11, 2001, authorization to the National Security Agency to carry out electronic surveillance into private phone calls and e-mails is consistent with court decisions and with the positions of the Justice Department under prior presidents.

The president authorized the NSA program in response to the 9/11 terrorist attacks on America. An identifiable group, Al Qaeda, was responsible and believed to be planning future attacks in the United States. Electronic surveillance of communications to or from those who might plausibly be members of or in contact with Al Qaeda was probably the only means of obtaining information about what its members were planning next. No one except the president and the few officials with access to the NSA program can know how valuable such surveillance has been in protecting the nation.

Go ahead and read the whole thing. You'll also find:

The passage of the Foreign Intelligence Surveillance Act in 1978 did not alter the constitutional situation. That law created the Foreign Intelligence Surveillance Court that can authorize surveillance directed at an "agent of a foreign power," which includes a foreign terrorist group. Thus, Congress put its weight behind the constitutionality of such surveillance in compliance with the law's procedures.

But as the 2002 Court of Review noted, if the president has inherent authority to conduct warrantless searches, "FISA could not encroach on the president's constitutional power."

Think this is only something that Chimpy Bushhitler wanted to do? Think again.

Every president since FISA's passage has asserted that he retained inherent power to go beyond the act's terms. Under President Clinton, deputy Atty. Gen. Jamie Gorelick testified that "the Department of Justice believes, and the case law supports, that the president has inherent authority to conduct warrantless physical searches for foreign intelligence purposes."

Before you dismiss this as the writings of some conservative neocom/nazi, check the author's certs at the bottom of the article.

John Schmidt served under President Clinton from 1994 to 1997 as the associate attorney general of the United States. He is now a partner in the Chicago-based law firm of Mayer, Brown, Rowe & Maw.

So, are you suggesting we impeach Bush for following the law?