I have been setting up a cassandra cluster for logging. One of the things that annoyed me was that the Debian packages do not include the cqlsh tool for interacting with the servers. So here is the way I installed it for cassandra-1.0.10. Note that the cqlsh in cassandra-1.1.0 works out of the box, but does not connect to 1.0 servers.
We're going to install the required python libraries using easy_install so first we need to python-dev packages for building and setuptools which includes easy_install. Then just install cql.
apt-get install python-dev python-setuptools