Question

I couldn’t find this documented in Magento Wiki and I didn’t know exactly what database table or class in the core/ folder it lies in.

Answer 1

Load the object and use getMetaKeyword() on it. Note that categories use getMetaKeywords()

$_product = Mage::getModel('catalog/product')->loadByAttribute('sku',$sku))

$_product->getMetaKeyword();

A faster method to load the object since SKU is a static attribute is to use

$_product = Mage::getModel('catalog/product')->load($sku,'sku');

$_product->getMetaKeyword();