form validation for NOT matches

[eluser]blorriman[/eluser]
I have a form and I am using the following to create an input:
echo form_input('first_name', set_value('first_name', 'enter first name'));

I then validate the form:
$this->form_validation->set_rules('first_name', 'Name', 'trim|required');

However, because the form opens with "enter first name" in the text input, the required validation doesn't work.

I like the look this way instead of adding a label, but how do I validate this as required now?