In case you haven't seen it yet, Apache released Cassandra 1.2:
I'm stoked. Presently we write maps/lists as text blobs in Cassandra using JSON. This has obvious limitations. In order to add something to the map/list, we need to read the list, append the data then write the data back to C*. Read before write is not good, especially in a large distributed system. Two clients could read at the same time, append, and the second write would effectively remove the element added by the first. Not good.
Although I think the Thrift support is a bit clunky (via JSON), CQL3 supports native operations on collections.
Now, we just need to figure out how to migrate all of our data. =)
For more information see: http://www.datastax.com/dev/blog/cql3_collections