From the news:
Lucene can be downloaded from http://www.apache.org/dyn/closer.cgi/lucene/java/5.1.0 and Solr can be downloaded from http://www.apache.org/dyn/closer.cgi/lucene/solr/5.1.0
Both releases contain a number of new features, bug fixes, and optimizations.
See the Lucene CHANGES.txt and Solr CHANGES.txt files included with the release for a full list of details.
See also Solr 5.1 Features by Yonik Seeley.
Of particular interest, Streaming Aggregation For SolrCloud (new in Solr 5.1) by Joel Bernstein.
Enjoy!