Change User Registration Form Fields

on the user registration form I have enable Profile and added a 'First Name' and 'Last Name' field. They are in the array under 'Personal Information'. I would like to have a deafult value of First Name in a light gray in the box rather than a label above, I read through 11heavens ad some other tutes and they talk about adding a terms and conditions but not how to modify the array vaulues for #default_value or maybe there is an easier way? here is what I am trying:

  /**
   * Theme override for user edit form.
   *
   * The function is named themename_formid.
   */
  function hglitigation_user_register_form($form) {
    // Our kickin' mods go here.
	 $output = '';
  // Print out the $form array to see all the elements we are working with.
  //$output .= dsm($form);
  // Once I know which part of the array I'm after we can change it.

  // You can use the normal Form API structure to change things, like so:
  // Change the Username field title to Login ID.
  $form['Personal Information'']['profile_first_name']['#default_value'] = t('First Name');

  // Make sure you call a drupal_render() on the entire $form to make sure you
  // still output all of the elements (particularly hidden ones needed
  // for the form to function properly.)
  $output .= drupal_render($form);
  return $output;
  }