I do not recommend Cassandra 2.01 unless you need to develop around the lightweight transactions or other features it offers. DataStax enterprise version of Cassandra is based on Cassandra 1.2.x currently, but is extremely well tested and they offer enterprise-level support.
I found their instructions very useful in transitioning my systems to DSE Cassandra: http://www.datastax.com/documentation/cassandra/1.2/webhelp/index.html#cassandra/install/installCassandraTOC.html
Cassandra 2.01 installed!
Instructions for ubuntu-server 12.04:
add “deb http://www.apache.org/dist/cassandra/debian 20x main” to /etc/apt/sources.list
gpg –keyserver pgp.mit.edu –recv-keys 4BD736A82B5C1B00
gpg –export –armor 4BD736A82B5C1B00 | sudo apt-key add –
apt-get install cassandra