"While Lucene has long offered these capabilities, its native capabilities are not intended for large semi-structured document collections (or documents with very different schemas)," the project website says. "For this reason we developed SIREn - Semantic Information Retrieval Engine - a Lucene plugin to overcome these shortcomings and efficiently index and query RDF, as well as any textual document with an arbitrary amount of metadata fields."
From Sindice's announcement: "The difference between SIREn and any of your well known RDF databases is that it uses the same principles found in web search engines rather than techniques derived by the database world, namely it is an Information Retrieval based engine."
Sindice is using SIREn as its new front-end - you can test drive it here.
Sindice also released a new version of its search API.