Drupal 7: display username in user-menu block title

template.php

<?php

/**
 * Implements MY_THEME_preprocess_block().
 */
function MY_THEME_preprocess_block(&$variables) {
  global $user;
  $block =& $variables['block'];

  if ($block->module == 'system' && $block->delta == 'user-menu' && user_is_logged_in()) {
    $block->subject = check_plain($user->name);
  }
}