Cassandra encryption at rest
Great question! There's a handful of
ways of doing this, depending on how deeply you want to be involved in
the configuration and long term maintenance.  You certainly could use
Cassandra on top of an encrypted filesystem, such as TrueCrypt or
eCryptfs, and it should function normally.

While possible, it's a little complicated to set up encrypted
filesystem and enforce a secure key management scheme.  Gazzang offers
a commercial product built on top of eCryptfs which provides a turnkey
Encryption &amp; Key Management Platform that would keep all of your
Cassandra data secure, without requiring any changes to your existing
database applications.  Gazzang has a custom-tailored "jumpstart"
documentation for Cassandra that simplifies and assist in your
encryption/security implementation for Cassandra.

Full disclosure: I am employed by Gazzang.