I think people have been doing these things for years, in the past they were just more apt to be embedded in desktop applications and so forth. Issues with scaling for the web have changed the dynamics, so the new non-relational approaches are quite distinct from the earlier ones such as Statice, ObjectStore, and Ontos.

CouchDB is well worth a hard look mainly because it takes advantage of several new ideas all in a very simple stack.

In a year or two I predict two or three will emerge as clear choices for a few distinct scenarios.