http://hadoop.apache.org/hive/ is a project that runs SQL queries against Hadoop map/reduce clusters. (For analytics; it is too high-latency to run applications against Hive directly).
HIVE-705 added support for backends other than HDFS, with HBase as the first. Cassandra support should be doable too now.
The Hive storage backends are described in http://wiki.apache.org/hadoop/Hive/StorageHandlers and the HBase backend specifically in http://wiki.apache.org/hadoop/Hive/HBaseIntegration.
I also note that John Sichi, author of the HBase backend, seems like a helpful guy and I imagine would be totally cool with answering questions about implementation details.