I couldn’t find the original Jena announcement, but here’s an early paper from Brian McBride: Jena: Implementing the RDF M+S Specification from Brian McBride on 2001-03-08 (www-rdf-interest@w3.org from March 2001).
5 years later, Jena 2.4 ships with an ampressive feature list, and an even more impressive user community…