The Northeastern University study--according to this Associated Press story--could only have done its secret tracking because it was conducted outside of the United States, because such testing would likely be illegal here. Retailers looking to try out new mobile strategies might consider testing the domestic-only applications overseas.
Secret Cellphone Tracking Study Raises Non-U.S. Mobile Retail Potential
A university study of cellphone users' habits--which found that nearly half of the people tracked kept to a circle little more than six miles wide--is raising some ethical issues and might suggest a non-traditional retail testing method.