new-product.php

<?php
public function isSalable()
{
    //function modified to store answers in the magento registry
    if (is_null(Mage::registry('isSaleable' . $this->getId())))
    {
        Mage::dispatchEvent('catalog_product_is_salable_before', array(
            'product' => $this
        ));

        $salable = $this->isAvailable();

        $object = new Varien_Object(array(
            'product'    => $this,
            'is_salable' => $salable
        ));

        Mage::dispatchEvent('catalog_product_is_salable_after', array(
            'product' => $this,
            'salable' => $object
        ));

        Mage::register('isSaleable' . $this->getId(), $object->getIsSalable(), true);
    }

    return Mage::registry('isSaleable' . $this->getId());
}

old-product.php

<?php
public function isSalable()
{
    Mage::dispatchEvent('catalog_product_is_salable_before', array(
        'product'   => $this
    ));

    $salable = $this->isAvailable();

    $object = new Varien_Object(array(
        'product'    => $this,
        'is_salable' => $salable
    ));
    Mage::dispatchEvent('catalog_product_is_salable_after', array(
        'product'   => $this,
        'salable'   => $object
    ));
    return $object->getIsSalable();
}