I'm going to define a unified data model as a model of data architecture that is necessarily rooted in actual digital hardware (otherwise it would just be a Data Model or Object Model) that can encompass every data relationship and scale arbitrarily large.
Such an idea has been proposed before, in jest by John Hodgeman and seriously by Dan Brickley, for instance here: danbri.org 
Hmm, nothing found on that link.
Somewhere on this wiki it's pointed out that tools are often defined by their limits as well as what they can do. Standards impose limits, otherwise they won't be standards. It could be called a form of discipline or regimentation in order to "tame" things. The trick is to find something that's flexible and can cover a lot of domains, yet not be so open-ended that it's RAM-like mush.
Yes, but the idea of a unified data model is to impose a standard that is universal enough to encompass a large scope of data and types of data. This is in contrast to an adhoc data model concocted to solve business problems, even for the enterprise.
Of course, for a such a unified model to scale and to be navigable, we need a Three Dimensional Visualization Model.
In theory this could be done with Endeme Sets using a Knowledge Database and a clever Endeme Path structure. I'm taking a shot at it, mostly so that I can create a game that regenerates universes once time travelers have affected the time stream. -- Jon Grover
See also Unified Object Model, Pangaia Project
See original on c2.com