Set NOINDEX, NOFOLLOW on particular products

In Magento admin, when editing a product go to the tab design and add the following to custom layout:

<reference name="head">
      <action method="setRobots"><meta>NOINDEX,NOFOLLOW</meta></action>
</reference>

you can observe the event controller_action_layout_generate_blocks_after like this:

public function setRobots($observer) 
{
     $controller = $observer->getAction();
     $fullActionName = $controller->getFullActionName();
     if ($fullActionName == 'catalog_product_view') { //if on product page
          $product = Mage::registry('product'); //access the current product if needed
          if (your condition here) { //condition to set the robots to noindex, nofollow
              $observer->getLayout()->getBlock('head')->setRobots('NOINDEX,NOFOLLOW');
          }
     }
}