The police are increasingly getting search warrants for information about all cell phones in a certain location at a certain time:
> Police departments across the country have been knocking at Google's door for at least the last two years with warrants to tap into the company's extensive stores of cellphone location data. Known as "reverse location search warrants," these legal mandates allow law enforcement to sweep up the coordinates and movements of every cellphone in a broad area. The police can then check to see if any of the phones came close to the crime scene. In doing so, however, the police can end up not only fishing for a suspect, but also gathering the location data of potentially hundreds (or thousands) of innocent people. There have only been anecdotal reports of reverse location searches, so it's unclear how widespread the practice is, but privacy advocates worry that Google's data will eventually allow more and more departments to conduct indiscriminate searches.
Of course, it's not just Google who can provide this information.
I spend a lot of time talking about this sort of thing in Data and Goliath. Once you have everyone under surveillance all the time, many things are possible.