1. A method comprising, at a mobile device: transmitting one or more indicators of a location of the mobile device to a server; receiving from said server parameters representing an inference model based, at least in part, on the one or more indicators of said location; determining that said mobile device is located in or approaching a particular region of a plurality of candidate regions based, at least in part, on application of one or more observations obtained by said mobile device to said parameters representing said inference model; transmitting a request message to said server indicating that said mobile device is located in or approaching the particular region; and receiving from said server positioning assistance data determined based, at least in part, on said request message.