/**
 * Implements hook_menu_alter().
 */
function MYTHEME_menu_alter(&$items) {
  $items['user/%user']['title callback'] = 'MYTHEME_user_page_title';
}

/**
 * Custom Callback for setting page title.
 */
function MYTHEME_user_page_title() {

  if (arg(0) == 'user') {
    // Load uid from url
    $user       = user_load(arg(1));
    $wrapper    = entity_metadata_wrapper('user', $user);
    $first_name = $wrapper->field_profile_first_name->value();
    $last_name  = $wrapper->field_profile_last_name->value();
    if ($first_name && $last_name) {
      drupal_set_title('User Profile - ' . $first_name . ' ' . $last_name);
    }
    else {
      drupal_set_title('User Profile');
    }
  }
}