Funny you should ask.
To give you some perspective on how secret the NSA is, consider this: For a year, The New York Times, which had published the Pentagon Papers, sat on the story of NSA violating the rights American citizens.
Not quite a year ago, the President had nominated John Bolton to be the US Ambassador to the United Nations. In April, his nomination hit a snag when it was revealed that he wanted the names of some U.S. citizens that had been blacked out of NSA intercepts he had requested.
At the time of his nomination, Bolton was the Under-Secretary of State, dealing with non-proliferation. He had had his run-ins with Colin Powell, then Secretary of State and it has been widely speculated that he was spying on the boss. This is important because if true, it shows that the NSA was engaged in domestic spying for political purposes.
We may never know if Bolton was bugging his boss, though Powell's willingness to speak out about W's lame defense of Snoop-gate, suggests that he has his own opinion on this.
Here's what we know so far: Bush was breaking the law to snoop on Americans. It is only logical to assume that the reason the Bushies didn't go to the FISA court is because they knew it wouldn't grant their request for authorization.
What kind of request would the FISA court turn down? I suppose one kind of snooping that they would frown upon would be snooping for purely political purposes. Can you think of another?
" ... tell 'em Big Mitch sent ya!"