An excellent question. So many of the kneejerk civil-rights absolutists have made the assumption that warrantless wire taps are unusual or totally illegal.
Powerline blog has an excellent analysis on the legality of the President's actions. Long - but definitive and worth the time. He's on strong legal ground - both in law, precedent and history!
http://powerlineblog.com/archives/012631.php
Title: On the Legality of the NSA Electronic Intercept Program
One allegation has been made which, if true, could be a violation and that is the claim that the administration submitted requests to the FISA court, were turned down, and then tapped them anyway. As for other violations, if any, I don't know. But enough claims have been made to warrant a legal investigation. If Bush is right then the courts need to uphold him. And if he's wrong then we need to know that, too. Nobody, not even the President, can act outside the law.