In Django 1.1 you could customize the error messages for unique constraints over multiple fields by overriding unique_error_message in your form. In Django 1.2 much of the validation has been moved to the model, including the unique_error_message checks.

Using Django 1.1 you'd use following code in your form class:

def unique_error_message(self, field):  
    if field == ('field1', 'field2'):  
        return "You should not do that"  
    return super(MyForm, self).unique_error_message(field)

In Django 1.2 you'll have to change this to the following code in your model class:

def unique_error_message(self, model_class, unique_check):
    if unique_check == ('field1', 'field2'):
        return "You should not do that"
    return super(MyModel, self).unique_error_message(model_class, unique_check)