The President has Constitutional authority to order surveillance without warrant given certain conditions of national security. This authority is above the 1978 FISA.
Quote:
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In a Sept. 25, 2002, brief signed by then-Attorney General John D. Ashcroft, the Justice Department asserted "the Constitution vests in the President inherent authority to conduct warrantless intelligence surveillance (electronic or otherwise) of foreign powers or their agents, and Congress cannot by statute extinguish that constitutional authority."
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http://www.msnbc.msn.com/id/10509407/page/2/
Therefore, the surveillance ordered by the President is lawful - as long as it was for reasons of National security.
It’s a broad power with potential for abuse. It’s scary, but it's that way for a very good reason: "to defend the national security of the United States against all enemies, foreign and domestic".
The President is upset about the leak because it compromises an ongoing investigation into terrorism operatives in the U.S.. Now that the terrorist groups have been made aware that they may be being monitored, they can use simple means to “work around” the surveillance.
TG posted an interesting link on the counter-measures subject a while back:
http://www.crypto.com/papers/wiretapping/
The leak places U.S. citizens in increased danger as terrorists could use counter-measures against surveillance and therefore escape detection.