I just tried to run the cassandra beta 3, and I had some problems on startup due to some missing jars.
Running ant jar fixed the problem for me. It downloaded all the dependencies, but it did not finish the build. <http://www.apache.org/dyn/closer.cgi?path=/cassandra/0.6.0/apache-cassandra-0.6.0-beta3-bin.tar.gz> -- Tom Chen