Rich Hickey (of [Clojure][1] fame) has released a cloud-based database called [Datomic][2] that has some interesting properties.
Datomic is an log of assertions and retractions of "facts", much as a DVCS like [Git][3] is a log of code diffs. The state of the database at any one time is the sum of all the assertions and retractions up to that date.