Passing instance objects between controller and model

Are there any gotchas to creating standard PHP classes and passing instances of them between controllers and models? I don't mind putting include_once(APPPATH.'models/Some_Php_Class.php'); at the top of my controller if it means I can group posted items into a single self-documenting object that has it's own validation methods, etc. that I can pass to the model. Or equally, I would like to pull a collection of object instances into the controller and build selection arrays based on a criteria.

Could I run into concurrency issues with this practice?