According to
Mage::app()->getWebsite(true)->getDefaultStore()->getRootCategoryId();
ID of the default product category is '2'
but:
public function initProduct(Varien_Event_Observer $observer)
// catalog_product_new_action event
{
$product = $observer->getEvent()->getProduct();
(...)
$productCatalog = Mage::getModel('catalog/product')->load($product->getId());
$productCatalog->setCategoryIds(array("2"));
$productCatalog->save();
$product->save();
(...)
doesn't change product's category at all. It's either wrong ID or wrong way of changing the category.