California's state legislature passed the Location Privacy Act of 2012 (SB-1434) on Wednesday, which would make it mandatory for law enforcement agencies to obtain a warrant before gathering any GPS or other location-tracking data that a suspect's cell phone might be sending back to its carrier.

The act, sponsored by State Senator Mark Leno (D-San Francisco), and co-sponsored by the ACLU of California and the EFF, passed with strong support by both parties. It now needs to be signed by California's governor to be put into law. But the bill isn't necessarily a straight-shot to penal code glory: the EFF points out that Governor Jerry Brown vetoed California's last attempt at enforcing stricter privacy rules in 2011, when he killed a bill that would have prevented police from searching the phones of apprehended suspects without a warrant.

Earlier this year, the ACLU requested documents from 380 law enforcement agencies nationwide, and found that responses from over 200 local law enforcement agencies showed "that while cell phone tracking is routine, few agencies consistently obtain warrants. Importantly, however, some agencies do obtain warrants, showing that law enforcement agencies can protect Americans' privacy while also meeting law enforcement needs."

Last week the subject of warrantless GPS tracking came up again in the national news, when a federal appeals court ruled that law enforcement is allowed to track the GPS signal coming from a suspect's prepaid phone without a warrant. Privacy advocates questioned the constitutionality of that ruling, which seems to open the door to a world where cellphones can reveal much more information to the wrong people than their users would ever have expected.