In a recent (youtube) talk, see link below twitter demonstrated the smart use of Sql (MySql) and no-sql ( Hadoop, Cassandra , Vertica) technologies.

Twitters sql architecture is a distributed data storage framework built on top of MySQL (InnoDB).
InnoDB was chosen because it doesn't corrupt data. There are some performance tweaks like disabling binary logs and replication.

Other no-sql technoglogies are added to the mix to act as speed and scalability enablers. Cassandra is used for high velocity writes, and the fact that it can scale out easier on cheap hardware. Hadoop is used to process unstructured and large datasets without having first to have a schema.

Vertica is being used for analytics and large aggregations and joins so they don't have to write MapReduce jobs.

The youtube talk is at http://www.youtube.com/watch?v=5cKTP36HVgI