I have Magento 1.3.2 and I have weird issue:
When I am in
The other option is to re-load the product object in the list.phtml using the ID of the product you already have. The code reads something a little like:
$sku = Mage::getModel('catalog/product')->load($_product->getId())->getSku();
Note that $_product is what you are getting in your collection already, and note that getSku is case sensitive (as are all Magento attributes getter/setters).
@Prattski's solution is preferable as you don't really want to be messing with loading/manipulating the objects, but it sounds as though your collection is a little messed up. SKU is one of the core fields that exists in the base
catalog_product_entity table, so would be unusual not to be loaded.