Is the W3C or any standards body working on a single standard for Entity Attribute ORM definitions?
To answer the question: no; not that I am aware of.
This being said, XForms doesn't much to do with ORM. With XRX, it can be seen as the opposite of ORM, the goal being to avoid mapping, and the complexity it creates. If you use the same data structure (XML in the case of XRX) to hold data all the way, from your UI, to the services you call, to your database, you avoid data transformation and mapping, reducing the complexity of your system.
There are literally dozens of XML, YAML, JSON, and nested array based (that whole convention over configuration thang) standards for describing : Database tables, Classes, Maps between Tables and Classes, Constraints, User Interface descriptions, Maps between Entities and User Interfaces, Rules for user Interfaces, etc.  Every major language has a system and competing standards. [http://en.wikipedia.org/wiki/List_of_object-relational_mapping_software]
Some "patterns" like Active-Record are getting implemented in PHP, Python, Ruby, Java etc.  But there is no single consensus XML or the nested array thingy de-dur.  mean while back in Redmond, Microsoft is crafting XML standards for, well, everything and now with the Entity Framework they have yet another ORM standard.  
Entity Framework + WPF (Windows Presentation Foundation) + WCF (Windows Communication Foundation) +  WF (Windows Workflow Foundation) + LINQ (language-integrated query) = ???
I recall Mozilla's XUL was a nifty thing, but it did not include ORM.  Seems like Microsoft is creating a massive set of standards, in XML, that can be used to define entire classes of applications from web, to mobile, to thin client desktop, to traditional heavy desktop app...all, incredibly...with a single set of standards.
So ... to conclude ... W3C has XForms ... but (we) need an ORM standard to move things along, something that can be implemented in PHP, Python, Ruby, Java, Objective C, Perl, Javascript, C++, and oh ya C#.  If it's active record...ok...fine...but I some how think that the problem is much bigger than Active Record can handle all by itself.