I have set up an observer on catalog_product_collection_load_after and the following code is called:

<?php
class Drench_Admindetails_Model_Observer {
    public function loadAfter($observer){
        $collection = $observer->getEvent()->getCollection();
        $collection->addAttributeToFilter('admin_id', Mage::getSingleton('admin/session')->getUser()->getUserId());
        foreach($collection as $item) {
            fb($item->getAdminId()); //fb() is a firebug call
        }
        return $this;
    }
}

As you can see, I am filtering the collection on admin_id, which I created through the following setup script (namespace/module/Resource/Eav/Mysql4/Setup.php).

<?php

class Drench_Admindetails_Resource_Eav_Mysql4_Setup extends Mage_Eav_Model_Entity_Setup
{
    public function getDefaultEntities()
    {
        return array(
            'catalog_product'                        => array (
                'entity_model'                       => 'catalog/product',
                'attribute_model'                    => 'catalog/resource_eav_attribute',
                'table'                              => 'catalog/product',
                'additional_attribute_table'         => 'catalog/eav_attribute',
                'entity_attribute_collection'        => 'catalog/product_attribute_collection',
                'attributes'                         => array (
                    'admin_id'                       => array (
                        'group'                      => '',
                        'label'                      => '',
                        'type'                       => 'int',
                        'input'                      => '',
                        'default'                    => '0',
                        'class'                      => '',
                        'backend'                    => '',
                        'frontend'                   => '',
                        'source'                     => '',
                        'global'                     => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_STORE,
                        'visible'                    => false,
                        'required'                   => false,
                        'user_defined'               => false,
                        'searchable'                 => false,
                        'filterable'                 => false,
                        'comparable'                 => false,
                        'visible_on_front'           => false,
                        'visible_in_advanced_search' => false,
                        'unique'                     => false
                    )
               )
           )
        );
    }
}

This attribute stores the admin that added the product. However, the collection is not filtered on admin_id, and in the foreach() loop in the observer method, it returns NULL rather than the actual admin_id it should return.

Any ideas on why its not working?

You cannot filter a collection after it has been loaded. Use the event catalog_product_collection_load_before instead. If you attempt to iterate the collection at this point it may call the same event and start an infinite recursion, which would be bad.

The admin_id attribute probably won't be added to the selected columns for product lists unless the attribute has used_in_product_listing set to true. You might also be successful with using $collection->addAttributeToSelect('admin_id') in the same before load event.