Where to put plain object classes?

@sneakyimp

Offtopic a little, anyway:

"I could extend CI_Model, I suppose, but why would I do this?" - an example: https://github.com/jamierumbelow/codeigniter-base-model

"What happens when my DataObject defines a property that already belongs to the CI singleton?" - I have never hit such case, but if it happens, it could be easily resolved. Most probable candidate is the 'config' property, so within the constructor you can write $this->ci_config = get_instance()->config;