I use Apache Solr for search on several projects, including a few with Drupal. Solr has built in support for replication and load balancing, unfortunately the load balancing is done on the client side and works best when using a persistent connection, which doesn’t make a lot of sense for PHP based webapps. In the case of Drupal, there has been a long discussion on a patch in the issue queue to enable Solr’s native load balancing, but things seem to have stalled.