We were getting close with the suggestions but the final solution came from the following:
// Add Pos Product Type
$pos_items =
Mage::getModel('eav/entity_attribute_option')->getCollection()->setStoreFilter()->join('attribute','attribute.attribute_id=main_table.attribute_id', 'attribute_code');
foreach ($pos_items as $pos_item) :
if ($pos_item->getAttributeCode() == 'pos_product_type')
$pos_options[$pos_item->getOptionId()] = $pos_item->getValue();
endforeach;
$this->addColumn('pos_product_type',
array(
'header'=> Mage::helper('catalog')->__('Pos Product Type'),
'width' => '100px',
'type' => 'options',
'index' => 'pos_product_type',
'options' => $pos_options
));