Find this code in product.tpl
Code:
<p class="descr">{$product.fulldescr|default:$product.descr}</p>
and comment it out with {* *}
Then find this code
Code:
<div class="clearing"></div>
</div>
{/capture}
{include file="customer/dialog_product.tpl" title=$product.producttitle|escape content=$smarty.capture.dialog}
and just before it (or after <div class="clearing"></div>) insert
Code:
<p class="descr">{$product.fulldescr|default:$product.descr}</p>
Yo may have to play with it before you get it to display just the way you want it