If you haven't read it, I'd really encourage you to. It's not a difficult or overly technical read.
http://i.a.cnn.net/cnn/2006/images/08/17/nsa.lawsuit.pdfFirst, the judge dismissed the Govt argument that the "state secret" privilege required the case to be dismissed. The Govt argued that, in order to prove the basic elements of the claim, the ACLU (which I'll use as shorthand for plaintiffs) would have to have access to privileged state secrets, which are protected from disclosure by fed law. The Court said no to that argument because (and this was BRILLIANT on the ACLU's part) the whole case was based on public statements made by the Govt. The ACLU did no discovery at all, and proved the prima facie elements of their claims based solely on Administration statements made in public.
Then there's some technical standing arguments over whether ACLU had the right to sue, which of course the ACLU won. Whether or not they actually suffered an injury in fact might be a problem for the ACLU in the appellate court. I'm just speculating, but to me, that's the weakest part of their standing to sue.
The judge found the 4th Amendment violation pretty much based on the text of the amendment, in that the Foreign Intelligence Surveillance Act defines what is and is not reasonable in the context of the searches, and the Govt's failure to follow FISA procedure is tantamount to a 4th Am violation.
The best part is that the judge constantly throughout the opinion refers to the President as a creature of the constitution, created by Article II, and bound by the same. "There are no hereditary kings in America and no powers not created by the Constitution. So all "inherent powers" must derive from that Constitution." She's telling Shrub that the Oval Office only fits inside the Constitution, and no further, and that he'd better watch his step.