Automated natural language question answering systems are different from “search engines”. For me, “semantic” generally refers to the W3C specified “semantic web” technologies, which include DBpedia. Natural language, or “conversational”, *dialog systems* may or may not be “question answering systems” and may or may not be “semantic” per se.
As far as I know, there is currently no general purpose dialog system available based primarily on conventional “semantic web” technologies; however, I believe that Paul Allen‘s “Digital Aristotle” Project Halo  is or was heavily based on DBpedia. See my quick and dirty literature review on “DBpedia & Natural Language Agents 2011” .