per se unreasonable, subject only to a few specifically established and well-delineated exceptions'.” Katz v. United States, 389 U. S. 347, 357 ." (Arizona v Gant, 07-542, decided 4/21/09).
This was a case dealing with the 4th Amendment of the U.S. Constitution.
More specifically, it concerned the warrant-less search of a vehicle incident to an arrest.
Hey, isn't a wiretap and similar electronic surveillance a "search", and therefore a warrant-less "wiretap" is a violation of the 4th Amendment?
In Olmstead v US 277 U.S. 438 (1928), the U.S. Supreme Court said no, electronic surveillance is not a search.
In Katz v US, supra, in 1967 the US Supreme Court overruled Olmstead, and said, yes, electronic surveillance is a search.
Any such electronic surveillance without a warrant is per se unreasonable.
Thus, in the NSA electronic surveillance cases (which include AT&T, Verizon, and some other telecoms as defendants) the burden should be on the government to show why all those NSA wiretaps without warrants are legal.
Warrant-less wiretaps are per se illegal still according to today's ruling in the US Supreme Court.
UPDATE: See: ACLU vs. Clapper, Alexander, Hagel, Holder, and Mueller - 8