Distributed Lucene
This work has now been superseded by the Katta project
Katta project - http://www.sourceforge.net/projects/katta
Doug Cutting's original proposal: http://www.mail-archive.com/general@lucene.apache.org/msg00338.html
Also see
Bailey project - http://www.sourceforge.net/projects/bailey
Contrib for updating indexes using MapReduce - https://issues.apache.org/jira/browse/HADOOP-2951