class Namespace_Category_Model_Observer
{
    public function setMetaTitle(Varien_Event_Observer $observer)
    {
        $head = $observer->getLayout()->getBlock('head');
        $metaTitle = Mage::registry('current_category')->getMeta_title();
        Mage::log($metaTitle); //I get the right value
        if (Mage::registry('current_category')) {
            $head->setData('title', $metaTitle); //line 22
        }
    }
}