Here are some links discussing the Constitutionality of the Bush Administration's warrantless electronic surveillance program as well as related issues:
Congressional Research Service: Presidential Authority to Conduct Warrantless Electronic Surveillance to Gather Foreign Intelligence Information: "This memorandum lays out a general framework for analyzing the constitutional and statutory issues raised by the NSA electronic surveillance activity. It then outlines the legal framework regulating electronic surveillance by the government, explores ambiguities in those statutes that could provide exceptions for the NSA intelligence-gathering operation at issue, and addresses the arguments that the President possesses inherent authority to order the operations or that Congress has provided such authority."
US Department of Justice: Legal Authorities Supporting the Activities of The National Security Agency Described by the President: "As the President has explained, since shortly after the attacks of September 11, 2001, he has authorized the National Security Agency (“NSA”) to intercept international communications into and out of the United States of persons linked to al Qaeda or related terrorist organizations. The purpose of these intercepts is to establish an early warning system to detect and prevent another catastrophic terrorist attack on the United States. "
Morton H. Halperin (Open Society Institute/Center for American Progress) and Jerry Berman (Center for Democracy & Technology): A Legal Analysis of the NSA Warrantless Surveillance Program : "The government’s defense of the NSA program rests on both a claim of inherent powers and a claim of statutory authorization. This memorandum examines these arguments and concludes that they lack serious merit. It also explains why the administration’s end-run around FISA has not served the national security interests of the country and has undermined the civil liberties of the American people."
Peter Swire, Center for American Progress: Legal FAQs on NSA Wiretaps: "Based on the facts available to date, the wiretap program appears to be clearly illegal."
Orin Kerr: Legal Analysis of the NSA Domestic Surveillance Program: "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."
All Volokh Conspiracy posts on Warrantless Wiretapping
A group of prominent Constitutional Law professors wrote a letter to key members of Congress: "Although the program’s secrecy prevents us from being privy to all of its details, the Justice Department’s defense of what it concedes was secret and warrantless electronic surveillance of persons within the United States fails to identify any plausible legal authority for such surveillance. Accordingly the program appears on its face to violate existing law. "
John Markoff, The New York Times: Taking Spying to Higher Level, Agencies Look for More Ways to Mine Data: "A small group of National Security Agency officials slipped into Silicon Valley on one of the agency's periodic technology shopping expeditions this month. On the wish list, according to several venture capitalists who met with the officials, were an array of technologies that underlie the fierce debate over the Bush administration's anti-terrorist eavesdropping program: computerized systems that reveal connections between seemingly innocuous and unrelated pieces of information."
Matthew Segal, FindLaw's Writ: Why the Bush Administration's Legal Stance on "Don't Ask, Don't Tell" Undermines Its Legal Stance on the NSA's Warrantless Wiretapping: " If Bush truly believed that he had both the legal power and the obligation to make security the country's first priority, he would have attempted to scrap 'Don't ask, don't tell.'"
Yesterday, the Senate Judiciary Committee held a hearing: Wartime Executive Power and the NSA's Surveillance Authority II
Marty Lederman discusses a bill introduced into the Senate by Judiciary Committee Chair Arlen Specter: Mother of Mercy, Is This the End of FISA?!* "This bill would appear to do absolutely nothing to address whether the current and ongoing program(s) is (are) permisisble under current law -- that is to say, it would not seek to facilitate judicial review of the AUMF and Article II arguments on which the Administration is relying."
AP: Lawsuit Alleges Illegal Wiretaps by NSA: "Civil rights attorneys have sued the National Security Agency, claiming it illegally wiretapped conversations between the leaders of an Islamic charity that had been accused of aiding Muslim militants and two of its lawyers."
More links are available through the Wikipedia entry: NSA Warrantless Surveillance Controversy