1. In a data communications network having a plurality of mobile personal information devices, a computer-implemented method of providing a place-and-people based information service, the method comprising: receiving reports of locations of mobile personal information devices operated by a user and by people identified on a list for the user, wherein identifying information for each identified person on the list is accessible by the user prior to receiving the reports of locations, and wherein the presence of each identified person on the list indicates previous consent by the respective identified person to share information with the user via the place-and-people based information service; and using an inference engine to determine based at least in part on the reported locations which of the identified people on the list are within a specified proximity of the user, wherein the specified proximity is independent of geographic distance between the user and the identified people transmitting information indicating which of the identified people are within the specified proximity of the user to the user's mobile personal information device.