Cassandra is quite interesting – and time sync seems increasingly critical to correct operation. Here are some resources:
A paper on the storage model from developers at Facebook.
A reasonably clear introduction from IBM Developer Works.
A big difficulty for people who are, like me, more familiar with the relational databases is that Cassandra and similar DBs are concerned about a very different kind of computation/storage platform and IT environment (different kind of technical user/programmer).
Cassandra