The Inside Story of Why Congress Is Now Willing to Amend FISA:
Newsweek has the story here. The key news is that after the Administration started submitting TSP surveillance to the FISA court, one of the FISA judges objected to parts of it:
[A]t some point after the new program began, one of the FISA judges—who, by rotation, was assigned to review the program for periodic updates—concluded that some aspects of the warrantless eavesdropping program exceeded the NSA's authority under the Foreign Intelligence Surveillance Act, the basic 1978 law that governs eavesdropping of espionage and terrorist suspects, said the lawyer who had been briefed on the ruling. The judge refused to reauthorize the complete program in the way it had been previously approved by at least one earlier FISA judge, the lawyer said, adding that the secret decision was a "big deal" for the administration.
  What is the legal issue? The Los Angeles Times reports:
[O]fficials said the ruling's reach [affects] cases "where one end is foreign and you don't know where the other is" — meaning warrants would be required even when it was unclear whether communications were crossing the United States or involved a person in the United States.

One official said the issue centered on a ruling in which a FISA court judge rejected a government application for a "basket warrant" — a term that refers to court approval for surveillance activity encompassing multiple targets, rather than warrants issued on a case-by-case basis for surveillance of specific terrorism suspects.

"One FISA judge approved this, and then a second FISA judge didn't," the official said, speaking on condition of anonymity because the activities of the FISA court are classified.
  From what I can tell, the legal issue is how FISA applies when the government wants to monitor someone outside the United States whose communications happen to be routed through the United States through U.S. switches. And specifically, what to do when the government has no idea where the people are on the other end of that person's calls and e-mails, but they want to monitor from the U.S. switch. The problem, as I emphasized in my draft article Updating the Foreign Intelligence Surveillance Act, is that the government often won't know where people are or who they are -- and if the law requires them to know that, then how the law applies will be very hard to figure out.

  Specifically, the key legal issue here probably is the scope of 50 U.S.C. 1801(f)(2), which states that a FISA warrant is required if the government acquires "the contents of any wire communication to or from a person in the United States, without the consent of any party thereto, if such acquisition occurs in the United States." Under this statute, no FISA warrant is required so long as a communication is foreign to foreign. But what happens if no one knows if a communication is foreign to foreign?

  To pick an isolated example, imagine the government is monitoring Bad Guy #1 in Pakistan, outside the United States, and Bad Guy #1 gets 10 phone calls and 30 e-mails every day from lots of different anonymous phone numbers and e-mail addresses. Can the government assume that the calls and e-mails are from places other than the U.S. and monitor the calls without a FISA order? Or must the government presume that the calls and e-mails are from inside the U.S. unless proven otherwise? What kind of proof is sufficient either way?

  I gather the disagreement between the two FISA judges is what to do in this setting. Off the top of my head, the statute is probably vague enough that one judge could reasonably say that the unintended collection of information from a non-target that turns out to be in the U.S. is permitted, whereas another judge could reasonably say that the government needs to provide some kind of assurance first that no such collection is likely to occur. And depending on what kind of assurance the judge wants, that assurance could be a major shift in what kind of monitoring the government can conduct. And that presumably explains the recent Congressional move to amend FISA to clarify what should happen in these settings.

  Anyway, those are my tentative thoughts. As always, we don't know all the facts: I'm just trying to give you my best guess as to what is happening and why based on the few facts that have leaked out. I look forward to your comments.