We've made some enhancements to the search infrastructure of the portal recently. We abstracted out Lucene as a search engine, now people can build plugins that integrate with others search engines frameworks.
For example, it's possible to replace Lucene with Solr, FAST and Google Search Appliance as long as a plugin is developed to interface with them.
Currently the only plugin developed is one for Solr, the Lucene plugin is the default implementation and comes bundled with the portal.
Roughly speaking, Solr is an application that listens for indexing and searching commands via HTTP requests. You can deploy Liferay in one server and have Solr running in a different server, this is very helpful for clustered portals because we got rid of racing issues when Lucene JDBC was used. Also, you can have a cluster of Solr instances that does load balancing of requests.
This works with Liferay 5.1.1, for more information about installing and running this plugin check these wiki pages:
(Thank you Rich for writing them!)
This plugin was tested with Solr 1.2 only. Looks like Solr 1.3 will be released soon, the plugin will be upgraded if necessary.