I've followed the discussion of the NSA's efforts with interest, both in the NYTimes and by Orin Kerr at Volokh.com. (Orin's a law professor who has mixed opinions about the legality of what Bush authorized. See his new discussion of James Risen's book (Risen being the reporter who got the story originally) and search the site for a number of interesting posts and comment threads.
Today the Post and Times report this development: Files Say Agency Initiated Growth of Spying Effort - New York Times: "The National Security Agency acted on its own authority, without a formal directive from President Bush, to expand its domestic surveillance operations in the weeks after the Sept. 11 attacks, according to declassified documents released Tuesday." Adm. Hayden briefed Congressional members and Rep. Pelosi challenged him on whether he had Presidential authority to broaden his eavesdropping after 9/11.
Perhaps influenced by Kerr, I tend to be somewhat sympathetic to the NSA (full disclosure--I passed their test in 1964 for employment, but the draft intervened). As I understand it, the Reagan administration in 1981 laid down the rules for what NSA could do in this area and the rules were followed until 9/11. Right after 9/11 NSA expanded their efforts on their own authority and then Bush authorized an expansion (not clear whether he authorized what NSA had already done, or whether it's separate expansions).
But the question that arises is, if the current expansions are legal and constitutional, why didn't the Reagan administration, no shrinking violet itself, authorize the steps in 1981? I have to think that Reagan thought Communism was a deadlier foe than most believe Al Qaeda to be, so it wasn't lack of motive. DCI Casey prided himself on breaking rules, so why play on a narrower field than you have to?
I suspect what happened is in the nature of bureaucracy. As my old calculus teacher would say, it's the "delta", the change. In 1981 we still had vivid memories of the 70's investigations into abuses by the CIA and FBI. Reagan's bureaucrats wrote the executive order grabbing all the authority they thought they could get away with. That became the normal order over the next 20 years. After 9/11, by definition the bureaucracy needed to do something more and different so they did. There's no way the bureaucracy could testify before Congress that they were doing "business as usual". One change (9/11) had to be met by another change.
Of course, another answer could be that in 1981 the bureaucrats thought today's authorities were unconstitutional or illegal under FISA.