In an amazingly short and to the point ruling (embedded below), a judge in a district court in Southern Texas, Lynn Hughes, ruled that letting the government get mobile phone data without a warrant was unconstitutional
When the government requests records from cellular services, data disclosing the location of the telephone at the time of particular calls may be acquired only by a warrant issued on probable cause. U.S. Const., amend. 4. The records show the date, time, called number, and location of the telephone when the call was made. These data are constitutionally protected from this intrusion. The standard under the Stored Communications Act, 18 U.S.C. § 2703(d), is below that required by the Constitution.
Of course, there are a number of cases out there that have ruled on similar issues... and come to different conclusions. This is one of those issues that will continue to bounce around until the Supreme Court clarifies. Still, there's something nice about seeing a court ruling of this nature, where the judge doesn't waste any time at all, and basically just says, "Hey, 4th Amendment! Next!"