Wide-area sensing services enable users to query data collected from multitudes of widely distributed sensors. In this paper, we consider the novel distributed database workload characteristics of these services, and present IDP, an online, adaptive data placement and replication system tailored to this workload. Given a hierarchical database, IDP automatically partitions it among a set of networked hosts, and replicates portions of it. IDP makes decisions based on measurements of access locality within the database, read and write load for individual objects within the database, proximity between queriers and potential replicas, and total load on hosts participating in the database. Our evaluation of IDP under real and synthetic workloads, including flash crowds of queriers, demonstrates that in comparison with previously-studied replica placement techniques, IDP reduces average  response times for user queries by up to a factor of 3 and reduces network traffic for queries, updates, and data movements by up to an order of magnitude.