I have been having similar problems when displaying my foreign key values in comboboxes.

I think that the problems relate to how these objects are registered with the form (and the binding context).
Must you regester these "lookup" business objects (used by comboboxes and pickers) as ChildObjects?
Would this assist in push error handling by Business Rules for these "lookup" business objects up to a parent object?

Could someone give a simple example for how to configure these "lookups" using comboboxes and pickers in order to pass the corresponding binding object source's business rules validation results to the form's error provider?