How do I override the look up on an entity reference autocomplete field? I would like to limit the look up to nodes of a certain type that have been created by the user that is creating the current node.
Create a view, and filter it to your needs. Then, add a new Entity Reference display. Configure this.
Then, on your content type(s), edit the entity reference field. Make sure you are using the AutoComplete widget. You should see a fieldset called Entity Selection. Change this from "Simple (with optional filter by bundle)" to "Views: Filter by an entity reference view". You should then see a dropdown appear which will have all of the views you have created that have Entity Reference displays. Pick your view and save.
Keep in mind, though, that views with large datasets can cause dramatic performance problems with autocomplete fields.