A new feature in Solr 4.0 is Optimistic Locking, along with RealTimeGet and Atomic_Updates it allows Solr to be used as a NoSQL Database. It allows a conditional update based on document version
field.
See http://wiki.apache.org/solr/Per%20Steffensen/Update%20semantics for more detail
http://yonik.com/solr/optimistic-concurrency/
https://issues.apache.org/jira/browse/SOLR-3178