Where to put plain object classes?

Because it would become your base model with the commons functions available to all your models.

You apparently missed this in my prior post:

All it does is write a log message when constructed and then define a __get magic method that refers to the CodeIgniter singleton. I could be completely missing something, but I wonder why I would want to have my DataObjects coughing up properties of the CI singleton object as if those properties were its own? What happens when my DataObject defines a property that already belongs to the CI singleton?