Looks like you're using the wrong form field... here's my code, which is working in one of my applications:
Controller:
public $components = array(
'Auth'=> array(
'authenticate' => array(
'Form' => array(
'fields' => array('username' => 'email')
)
)
)
);
View:
echo $this->Form->create();
echo $this->Form->input('email');
echo $this->Form->input('password');
echo $this->Form->end(__('Sign in', true));
Please notice that I'm using a "email" input in my form, and not "username" which is wrong.