1. A computer-implemented process for integrating a specialized knowledge source into a general search service, comprising using a computer to perform process actions for: using a search engine interface to receive a search query having search terms from a user; providing multiple expert knowledge engines each configured to enable retrieval of expert search results; using a predictive model module configured to generate a predictive model that provides a measure of a likelihood that one or more of the expert knowledge engines will provide valuable information for the search query and selecting one or more of the expert knowledge engines having highest probabilistic likelihoods of providing valuable information in response to the search query; using a query reformulation module to automatically reformulate the search query into multiple reformulated queries having increased probabilities of being effectively used by the selected expert knowledge engines to return relevant search results, determining a probabilistic ranking for each reformulated query, and providing one or more of the highest ranked queries for use by one or more of the selected expert knowledge engine to retrieve expert search results; and using a display module configured to display the retrieved expert search results to the user.