SpellCheckComponent can be configured to automatically (re)build indices based on fields in Solr index when a commit or an optimize is done. See http://wiki.apache.org/solr/SpellCheckComponent
UPDATE: I edit this post after seeing the exchange about this topic on SOLR user list: the FileBasedDictionaryIndex needs a spellchecker.build command issued at least once (basically ignore the build / optimize trigger defined on solrconfig.xml)