Open up this file:
app/design/frontend/rwd/{THEME_NAME}/template/catalog/product/view.phtml
If it does not exist, copy it to your local theme from:
app/design/frontend/rwd/default/template/catalog/product/view.phtml
You'll find in this file, these lines:
<?php if ($_product->getShortDescription()):?>
<div class="short-description">
<div class="std"><?php echo $_helper->productAttribute($_product, nl2br($_product->getShortDescription()), 'short_description') ?></div>
</div>
<?php endif;?>
Remove the "nl2br" PHP call that is in there prior to fetching the short description:
<?php if ($_product->getShortDescription()):?>
<div class="short-description">
<div class="std"><?php echo $_helper->productAttribute($_product, $_product->getShortDescription(), 'short_description') ?></div>
</div>
<?php endif;?>