If you don't want Honeypot module as mentioned by Jimajamma, you can do it with Form API #attributes all right:
$form['pass'] = array(
'#type' => 'password',
'#title' => t('Password'),
'#maxlength' => 64,
'#size' => 15,
'#attributes' = array('style' => array('display:none;'));
);