what I ended up doing is in app/design/frontend/default/theme_name/template/catalog/product/list_random.phtml

doing something like:

<?php
$_categories=$this->getCurrentChildCategories();

$_category = $this->getCurrentCategory();
$subs = $_category->getAllChildren(true);
$result = array();
foreach($subs as $cat_id) {
    $category = new Mage_Catalog_Model_Category();
    $category->load($cat_id);
    $collection = $category->getProductCollection();
    foreach ($collection as $product) {
        $result[] = $product->getId();
    }

}
shuffle($result);
?>

this will get you an array of product id’s. You can loop through them and create products on the fly using:

<?php
$i=0;
foreach ($result as $_product_id){
    $i++;
    $_product = new Mage_Catalog_Model_Product();
    $_product->load($_product_id);
    //do something with the product here
}?>

then, create a static block in the cms with the following content

{{block type="catalog/navigation" template="catalog/product/list_random.phtml"}}

Finally, in the Catalog->Manage categories section, choose the category, then the display settings tab. Switch the display mode to “Static block and products” and then choose your block from the drop list.

Source: http://stackoverflow.com/questions/454462/how-to-get-products-from-a-particular-category-in-magento-ecommerce