As announced here, I put a Debian package together for Cassandra 0.3.0.
I don't have any (immediate )plans to upload a Cassandra package to the Debian archive, (this package isn't even policy compliant), so consider this unofficial and report any packaging bugs directly to me.
deb http://people.apache.org/~eevans/debian cassandra/ deb-src http://people.apache.org/~eevans/debian cassandra/
Enjoy.