A new chapter in Solr scalability has been opened with the addition of distributed search!

http://wiki.apache.org/solr/DistributedSearch

Distributed Search splits an index into multiple shards, and queries across all the shards, combining the results and presenting a single merged response that looks like it came from a single server.

Solr’s current implementation uses SolrJ (the solr java client) to talk to other Solr servers via HTTP, in two main phases. The first phase collects matching document ids and scores, as well as doing any requested faceting. The second phase retrieves the stored fields for selected documents, does highlighting, and may include additional faceting requests to nail down exact facet counts.