If i activate Flat Catalog Product, than Magento do not view sku on Product and category page with this code:
<?php echo $this->htmlEscape($_product->getSku()); ?>
This means that the SKU attribute is not loaded. It should be enough to edit the
sku attribute under
Catalog - Attributes - Manage Attributes. If you want to use it on the product page, set "Visible on Product View Page on Front-end" to "Yes". If you want to use it on the category page, set "Used in Product Listing" to "Yes".