Chris Date has an interview over on the O’Reilly site that’s well worth the time to read. A favorite quote, one of many:
To a first approximation, “object/relational” just means the domains over which relations are defined can be of arbitrary complexity. As a consequence, we can have attributes of relations–or columns of tables, if you prefer–that contain geometric points, or polygons, or X rays, or XML documents, or fingerprints, or arrays, or lists, or relations, or any other kinds of values you can think of. But this idea too was always part of the relational model! The idea that the relational model could handle only rather simple kinds of data (like numbers, and strings, and dates, and times) is a huge misconception, and always was. In fact, the term object/relational, as such, is just a piece of marketing hype … As far as I’m concerned, an object/relational system done right would simply be a relational system done right, nothing more and nothing less.