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reference:
http://www.expertwebadvisor.com/display-sku-on-the-product-listing-page-and-the-product-description-page-in-magento/


on  /template/catalog/product/view.phtml 

look for the title tags:

<div class="product-name">
    <h1><?php echo $_helper->productAttribute($_product, $_product->getName(), 'name') ?></h1>
</div>


and just below that or a place you choose , put:
<?php echo nl2br($_product->getSku()) ?>