<?php
/**
* Magento
*
* NOTICE OF LICENSE
*
* This source file is subject to the Academic Free License (AFL 3.0)
* that is bundled with this package in the file LICENSE_AFL.txt.
* It is also available through the world-wide-web at this URL:
* http://opensource.org/licenses/afl-3.0.php
* If you did not receive a copy of the license and are unable to
* obtain it through the world-wide-web, please send an email
* to license@magentocommerce.com so we can send you a copy immediately.
*
* DISCLAIMER
*
* Do not edit or add to this file if you wish to upgrade Magento to newer
* versions in the future. If you wish to customize Magento for your
* needs please refer to http://www.magentocommerce.com for more information.
*
* @category   design_default
* @package    Mage
* @copyright  Copyright (c) 2008 Irubin Consulting Inc. DBA Varien (http://www.varien.com)
* @license    http://opensource.org/licenses/afl-3.0.php  Academic Free License (AFL 3.0)
* @author        Nick Cron
* @authorweb  www.njcmedia.com
*/
?>
<?php
$_related = $this->getProduct();

// get the parent id to skip
$_parentid = $_related->getId();

if ($_related) {
// get collection of categories this product is associated with
$categories =$_related->getCategoryCollection()->addFieldToFilter('level','3')  ->setPage(1, 1)->load();

// if the product is associated with any category
if ($categories->count())
foreach ($categories as $_category)
{

?>

<div class="box-collateral box-up-sell">
    <h2><?php echo $this->__('Related Product') ?></h2>
<table cellspacing="0" id="upsell-product-table">

<tr>

<?php

$visibility = array(
Mage_Catalog_Model_Product_Visibility::VISIBILITY_BOTH,
Mage_Catalog_Model_Product_Visibility::VISIBILITY_IN_CATALOG
);

$products = Mage::getResourceModel('catalog/product_collection')
->addCategoryFilter($_category)
->addAttributeToSelect('small_image')
->addAttributeToFilter('visibility', $visibility);

// Limiting to In Stock
Mage::getSingleton('cataloginventory/stock')->addInStockFilterToCollection($products);

$products->getSelect()->order(new Zend_Db_Expr('RAND()'));
$products->getSelect()->limit(6);

// count the number of displayed products
$_i=0;

foreach ( $products as $productModel ){
$_related = Mage::getModel('catalog/product')->load($productModel->getId());
$_realtedid = $_related->getId();

// prevents displaying the same product your are already on
if ($_realtedid != $_parentid && $_i<7):

?>

<td style="padding-right:15px;">
<a href="<?php echo $_related->getProductUrl() ?>"><img src="<?php echo $this->helper('catalog/image')->init($_related, 'small_image')->resize(100) ?>" width="100" height="100" alt="<?php echo $this->htmlEscape($_related->getName()) ?>" /></a>
															
<?php echo $this->getPriceHtml($_related, true) ?>
<?php echo $this->getReviewsSummaryHtml($_related) ?>
</td>

<?php
// increment displayed products
$_i++;
endif;
}
?>

</tr>

<?php }
}
?>
</table>
<script type="text/javascript">decorateTable('upsell-product-table')</script>
</div>

http://www.njcmedia.com/2011/04/auto-populate-cross-sell-with-products-from-current-category-magento/