The Court said judges can require a warrant if they want to, but they don't have to. Considering these requests are made by the Government ex parte, meaning there's no one present to argue against granting the request, how many judges are likely to refuse on their own when a higher court has said they don't have to?

More background on the case and the issues is here.

It should be a no-brainer that when the Government seeks information about your location from your cell phone, they need a warrant based on probable cause, not some boiler-plate statement to the judge that the information is relevant to an ongoing investigation.

In my view, cell-site locator information, whether historical or prospective, intrudes upon users’ reasonable expectations of privacy. It turns the cell phone into a tracking device, and under the Fourth Amendment, a warrant is required. As law professor Susan Friewald wrote in her Amicus brief in the case:

As the Magistrate Judge persuasively presented [opinion here], CSLI may disclose to law enforcement agents that a cell phone user has attended an Alcoholics Anonymous meeting, sought AIDS treatment, or visited an abortion clinic. See Lenihan Order, 534 F. Supp. 2d at 586 & n.6. CSLI may divulge when and where a user gave confession, viewed an X-rated movie, or protested at a political rally. Knowledge that the government could keep track of such information could easily inhibit valuable and constitutionally protected activities.

The Magistrate Judge got it right when ruling:

An Electronic Device That Is Able and Used to Provide the Government With Movement/Location Information is a "Tracking Device", Communications From Which are (i) Expressly Excluded from the Definition of "Electronic Communications" Under the SCA and (ii) Not Pertaining to the Subscriber of an Electronic Communications Service Under the SCA

and

[T]his Court concludes that CSLI is a ommunication from an electronic device that permits the tracking of the movement of a person, is therefore expressly placed outside the scope of the electronic communications legislation of the SCA, and is not appropriately brought back into the scope of information which the Government may seek to obtain thereunder by any reasonable reading of §2703©. The Court emphasizes that the foregoing analysis rejects a distinction between historic and prospective CSLI for purposes of § 2703©.

...nothing in the language of § 2703(d) indicates that information requested by the Government is obtainable as a matter of course upon a showing of reasonable relevance to a criminal investigation. To the contrary, § 2703(d) provides that an Order for disclosure shall issue "only if" the Government shows that the information sough is relevant. It does not provide that such an Order shall issue "if" or "whenever" such a showing is made. Thus, under the plain language of the SCA, a showing of reasonable relevance is a necessary, but not necessarily sufficient, condition for issuance of an Order.

Also:

This Court concludes, as a matter of statutory interpretation, that nothing in the provisions of the electronic communications legislation authorizes it to order a CSP's covert disclosure of CSLI absent a showing of probable cause under Rule 41. And this interpretation is abundantly confirmed by consideration of the Constitutional principles at issue. For reading the statutes in the manner advocated by the Government would, as to at least a substantial portion of the information at issue, violate Americans' reasonable expectation of privacy in any cell-phone-derived information/records as to the physical movements/locations by authorizing ex parte disclosure of that information with no judicial review of the probable cause. It appears to this Court, from its review of current Fourth Amendment case law and Constitutional principles, that this information is entitled to the judicial-review protections afforded by a probable cause warrant and historically applied to movement/location information derived from a tracking device.

On the reasonable expectation of cell phone users, the lower court correctly observed:

[T]he Fourth Amendment prohibits unreasonable searches and seizures and, accordingly, the Government must generally demonstrate probable cause and obtain a warrant prior thereto. To trigger the Fourth Amendment's protections, the individual must have a subjective expectation of privacy in the object of the Government's search, and it must be one which society accepts as objectively reasonable.

The Court believes, based on common experience within the community: First, that Americans do not generally know that a record of their whereabouts is being created whenever they travel about with their cell phones, or that such record is likely maintained by their cell phone providers and is potentially subject to review by interested Government officials. And second, that most Americans would be appalled by the notion that the Government could obtain such a record without at least a neutral, judicial determination of probable cause.

The Third Circuit today:

In sum, we hold that CSLI from cell phone calls is obtainable under a § 2703(d) order and that such an order does not require the traditional probable cause determination. Instead, the standard is governed by the text of § 2703(d), i.e., “specific and articulable facts showing that there are reasonable grounds to believe that the contents of a wire or electronic communication, or the records or other information sought, are relevant and material to an ongoing criminal investigation.” 18 U.S.C. § 2703(d). The MJ erred in allowing her impressions of the general expectation of privacy of citizens to transform that standard into anything else. We also conclude that this standard is a lesser one than probable cause, a conclusion that, as discussed below, is supported by the legislative history.

On different occasions in the MJ’s opinion, the MJ referred to her understanding that the “relevant legislative history indicates that Congress did not intend its electronic communications legislation to be read to require, on its authority, disclosure of an individual’s location information . . . .” MJOp., 534 F. Supp. 2d at 610. We also have reviewed the legislative history of the SCA and find no support for this conclusion.

...Because we conclude that the SCA does not contain any language that requires the Government to show probable cause as a predicate for a court order under § 2703(d) and because we are satisfied that the legislative history does not compel such a result, we are unable to affirm the MJ’s order on the basis set forth in the MJ’s decision.

The Third Circuit ends up saying a warrant is optional:

Because the statute as presently written gives the MJ the option to require a warrant showing probable cause, we are unwilling to remove that option although it is an option to be used sparingly because Congress also included the option of a §2703(d) order.

...We again note that although the Government argues that it need not offer more than “specific and articulable facts showing that there are reasonable grounds to believe that the . . .information sought . . . [is] relevant and material to an ongoing criminal investigation, ” 18 U.S.C. § 2703(d), the MJ never analyzed whether the Government made such a showing. We leave that issue for the MJ on remand.

I also think the Third Circuit's approach of leaving it up to the individual Magistrate Judge whether to require a warrant based on probable cause invites judge shopping. In many districts, Magistrate Judges have "duty weeks" where they handle the new criminal cases and requests for warrants. The Government, once it knows which Magistrate Judges don't think a warrant is necessary, can just wait to file the request, using the lower standard and without making a probable cause showing, until those judges are on "duty week."

At least the Third Circuit called on Congress to clarify the statute with respect to cell site locator and GPS data. But don't get your hopes up. Nor do I think it matters much which party controls Congress. The Dems are just as lax now in restricting wiretaps as they were in 1996, and conservative Republicans, while slightly better on the issue, would probably draft a bill requiring a warrant -- except in drug cases and excluding suppression as a remedy unless the Government acted in bad faith. Pick your poison.