One argument that might be decisive in either direction is that “a cell phone is not a person”.
That is, that as with the RIAA’s efforts to sue people based on their IP number, if their system is open WIFI, there is no way to associate that IP with a particular computer, much less a human user. (N.B. Google’s new “Chrome” browser *does* try to establish that a particular computer, at least, is the one that is surfing, so it takes away some of this anonymity.)
However, the flip side of the argument is that the police do not need a warrant to track *property*, only people. This, rather lame argument is the justification of property forfeiture in drug cases, even if no person is arrested.
Clearly, tracking people without probable cause is a violation of the 4th Amendment, so the bottom line is will the court be honest, or use convoluted arguments to justify this violation.
Several years ago, when GPS was fairly new, I talked with the owner of a store that sold GPS receivers. He said that the FBI was always coming in and buying GPS receivers from him. They would turn one on and hide it under the plastic covering of a car's metal bumper. Then they would retrieve it a few days later and download the track that showed where the vehicle had been. It wasn't real-time tracking, but it gave them ideas about the suspect's associates.