I came across a recent Slideshare presentation entitled "Complex Event Processing W/Cassandra". prepared by Brian O'neill and Taylor Goetz, from Healthcare Market Science. It describes a project integrating Cassandra and Storm. This presentation analyzes previous failures using Hadoop and C* with aspectj.
Then it explains the architecture of the solution using Storm for the event processing part.
Interesting presentation. It would have been even more interesting if the presentation talked more about the actual application (there is one slide explaining in very general terms what their products are).
It seems that a lot is going on in the open source space.