it took me a while to figure this out. Should work for you, if you customy it for your needs.

To list products affected by a price rule, you must make your way over the rule-model.

Here is the code:

<?php
// we need the date to filter for the special price
    $dateToday = date('m/d/y'); 
    $tomorrow = mktime(0, 0, 0, date('m'), date('d')+1, date('y'));
    $dateTomorrow = date('m/d/y', $tomorrow);

// this gets the product collection and filters for special price;
// grabs products with special_from_date at least today, and special_to_date at least tomorrow
        $_productCollection = Mage::getModel('catalog/product')->getCollection()
            ->addAttributeToFilter('special_from_date', array('date' => true, 'to' => $dateToday))
            ->addAttributeToFilter('special_to_date', array('date' => true, 'from' => $dateTomorrow))
            ->load();

// this loads the standard magento catalog proce rule model
        $rules = Mage::getResourceModel('catalogrule/rule_collection')->load();

// read: if there are active rules
        if($rules->getData()) {
            $rule_ids = array(); // i used this down below to style the products according to which rule affected
            $productIds[] = array(); // this will hold the ids of the products

            foreach($rules as $rule) {
                $rule_ids[] = $rule->getId();
                $productIds = $rule->getMatchingProductIds(); // affected products come in here
            }

// merge the collections: $arr is an array and keeps all product IDs we fetched before with the special-price-stuff
            $arr = $_productCollection->getAllIds();
            if($productIds) {
                // if there are products affected by catalog price rules, $arr now also keeps their IDs
                $arr = array_merge($arr,$productIds);
            }

// we initialize a new collection and filter solely by the product IDs we got before, read: select all products with their entity_id in $arr
            $_productCollection = Mage::getModel('catalog/product')->getCollection()
                ->addAttributeToFilter('entity_id',array('in'=>$arr))
                ->load();
        }

== UPDATE ==

I added some comments to the code and renamed $skus to $productIds (which fits better).

If you are still getting errors, please let me know what's exactly happening. This code is tested for Magento v1.7.0.2.

Cheers
Simon

If you defined somewhere (not in the cart rules) free shipping, there is no way to change this (I would guess).

But if you defined free shipping via cart rule, you can define a new rule, which is higher prioritized, check if it matches and then just stop processing (Stop further rules processing)