Using cassandra instead of memcache?
It would be silly to replace memcached as a cache with Cassandra in most situations.
What companies like Digg are doing is, replacing the database+memcached pair with Cassandra: Cassandra provides both durable storage, and an integrated, high performance cache (the "row cache"). This prevents problems like memcached being populated from an out of date slave (what Twitter calls "potential consistency") and simplifies cluster management.