1. A system including an application program interface (API) embodied on one or more computer readable storage media, comprising: a first group of services for integrating a plurality of content repositories into virtual content repositories (VCRs) such that the plurality of content repositories appear and behave as a single content repository, wherein the first group of services include first functions for authorizing access to the plurality of content repositories, second functions for incorporating combined content of the plurality of content repositories into a hierarchical namespace including representing the plurality of content repositories as a hierarchical namespace of nodes under a single VCR root node, third functions for extending a VCR content model to represent information in the plurality of content repositories including sharing a common representation of combined content of the plurality of content repositories as the hierarchical namespace of nodes under a single VCR root node between the API and a content repository service provider interface (SPI) implemented by each of the plurality of content repositories, and fourth functions for traversing the hierarchical namespace incorporating combined content of the plurality of content repositories; a second group of services for manipulating information in VCRs; a third group of services for searching VCRs; and a fourth group of services for configuring VCRs; wherein the application program interface is compatible with the content repository service provider interface (SPI) that maps operations on the VCR content model to the plurality of content repositories.