"I think cell-tower positioning would generally be good enough. If you have thousands of users then the positioning errors will tend to cancel each other out. You probably don't need to update the position very often either, since most people spend most of their time not moving very fast or very far."
Seems like the place to put the accelerometer is on the cell tower. Power wouldn't be a problem on the tower. Plus, wouldn't putting it in my cellphone generate a slew of false alarms whenever I jog up the stairs to my office, run across the street, or any number of everyday activities that would cause my cellphone to get jostled?