In http://www.allthingsdistributed.com/2007/10/amazons_dynamo.html and http://www.allthingsdistributed.com/2008/12/eventually_consistent.html Amazon discusses the concept of "eventual consistency." Cassandra uses eventual consistency in a design similar to Dynamo.
Supporting session consistency would be useful and relatively easy to add: we already have the concept of a Memtable (see http://wiki.apache.org/cassandra/MemtableSSTable ) to "stage" updates in before flushing to disk; if we applied mutations to a session-level memtable on the coordinator machine (that is, the machine the client is connected to), and then did a final merge from that table against query results before handing them to the client, we'd get it almost for free.
Of course, the devil is in the details; thrift doesn't provide any hooks for session-level data out of the box, but we could do this with a threadlocal approach fairly easily.
CASSANDRA-569 has some (probably out of date now) code that might be useful here.