How to Delete Configurable Product Attributes

Here is what seams to work for me for to remove an attribute from the configurable product.
This is the scenario.
All the configurable products were created wrong with the attribute brand as a configurable attribute for about 50 configurable products having about 200 simple associated products.
All the simple products associated to a configurable attribute have the same brand. The idea is to remove brand from configurable attributes and assign it as a simple attribute to the configurable product with the value of one of the simple products.
Here is the code that does this. The code is ran one time only. It can be added in an upgrade script or a simple php file.

<?php
//==>this is required only if you use a simple php file
error_reporting(E_ALL | E_STRICT);
$mageFilename = 'app/Mage.php';
require_once $mageFilename;
Mage::setIsDeveloperMode(true);
ini_set('display_errors', 1);
umask(0);
Mage::app();
//<==

$brand = 'brand';
//get the attribute instance
$brandAttribute = Mage::getModel('eav/config')->getAttribute('catalog_product', $brand);
//if this attribute exists
if ($brandAttribute->getId()){
    //make the attribute apply to al types of products in case it's not
    $brandAttribute->setApplyTo(null);
    $brandAttribute->save();
    $resource = Mage::getSingleton('core/resource');
    //get an object with access to direct queries
    $connection = $resource->getConnection('core_write');
    //get all configurable products - you can specify additional filters here
    $collection = Mage::getModel('catalog/product')->getCollection()
        ->addAttributeToFilter('type_id', 'configurable');
    foreach ($collection as $product){
        //the configurable attributes are stored in the table 'catalog_product_super_attribute'
        //remove the attribute references from that table. 
        //The constraints will take care of the cleanup.
        $q = "DELETE FROM {$resource->getTableName('catalog_product_super_attribute')}
            WHERE attribute_id = {$brandAttribute->getId()} AND product_id = {$product->getId()}";
        $connection->query($q);
        //get the simple products in the configurable product
        $usedProducts =  $product->getTypeInstance(true)->getUsedProducts(null, $product);
        foreach ($usedProducts as $p){
            //identify the first simple product that has a value for brand 
            //set that value to the configurable product.
            if ($brandValue = $p->getData($brand)){
                Mage::getSingleton('catalog/product_action')
                    ->updateAttributes(array($product->getId()), array($brand=>$brandValue), 0);
                break;
            }
        }
    }
}

For the numbers listed above, this took about 15 seconds to run on my local machine (not a powerful one). I'm sure that this can be optimized. Most probably there is no need to get all simple products of a configurable product to get the brand value, but I didn't bother.

Like Tim said in him comment above ideally you don't want to be running load within a loop as this causes extra work and MySQL calls. (Making it "really slow")

I had a play around and I think I have a rough version of what your trying to achieve - get a count of configurable products who have at least 1 associated products that are in stock in category X?

May not be perfect, but should work and much faster too.

    <?php
//Update part 1 start
$eavAttribute = new Mage_Eav_Model_Mysql4_Entity_Attribute();
$code = $eavAttribute->getIdByCode('catalog_product', 'status');
//Update part 1 end

    $currentCat = Mage::registry('current_category');

    //Gets all simple products that belong to a configurable product
    $productCollection = Mage::getResourceModel('catalog/product_type_configurable_product_collection')
        ->setFlag('require_stock_items', true)
        ->setFlag('product_children', true);

    //Adds stock, status and visibility filters.
    Mage::getSingleton('cataloginventory/stock')->addInStockFilterToCollection($productCollection);
    $productCollection->addAttributeToFilter('status', 1);
    $productCollection->addAttributeToFilter('visibility', 1);

    //Filters by simple products who's parents are in the current category
    $productCollection->getSelect()->joinLeft(
        array('ccp' => 'catalog_category_product'),
        'link_table.parent_id=ccp.product_id',
        array('*'))->where('ccp.category_id = ?',$currentCat->getId())->group('parent_id');

//Update part 2 start
    $productCollection->getSelect()->joinLeft(
        array('statust' => 'catalog_product_entity_int'),
        'link_table.parent_id=statust.entity_id',
            array('value'))->where('statust.value = ?',Mage_Catalog_Model_Product_Status::STATUS_ENABLED)->where('statust.attribute_id = ?',$code);

//Update part 2 end

    $cnt = $productCollection->count();
echo $cnt;

    ?>