The Third Circuit allows the Government to acquire cell phone data without probable cause.

In Harvard Law Review (Volume 124 · April 2011 · Number 6):

The Stored Communications Act (SCA) articulates the standard
the government must meet to obtain electronic communications
records from phone companies. In addition to the traditional option
of obtaining a warrant by showing probable cause, § 2703(d) of the
SCA permits magistrate judges to grant court orders for acquisition of
these records if the government meets a lower standard by “offer[ing]
specific and articulable facts showing that there are reasonable
grounds to believe” that the records “are relevant and material to an
ongoing criminal investigation.” The government has often attempted,
with varying degrees of success, to use § 2703(d) to obtain
cell-site location information (CSLI), which uses a cell phone’s communication with cell towers to determine the approximate location of
an individual over time. Recently, in In re The Application of the
United States for an Order Directing a Provider of Electronic Communication
Service to Disclose Records to the Government, the Third Circuit held that § 2703(d) applies to CSLI and that magistrates may grant court orders to obtain CSLI when the government meets § 2703(d)’s “specific and articulable facts” standard. But the court also gave magistrates the power — “to be used sparingly” — to require the government to show probable cause and obtain a warrant for CSLI. The Third Circuit failed to clarify exactly how often magistrates may require a warrant and did not explain what factors magistrates should balance in order to make this determination. As a result,
In re Application provides little guidance to magistrates about how often
and in what circumstances they may deny § 2703(d) orders.

Link:
http://www.harvardlawreview.org/media/pdf/vol124_in_re_the_application.pdf