Mailchimp Ecommerce 360 for Magento Order Event Observer .... Some Code from ebizmart's extension.... but I just needed the Ecomerce360 aspect not all the other stuff there extension provides.

config.xml

<?xml version="1.0" encoding="utf-8"?>
<config>
    <modules>
        <Somecompany_Extensionname>
            <version>0.1.0</version>
        </Somecompany_Extensionname>
    </modules>
    <global>
        <models>
            <myobserver>
                <class>Somecompany_Extensionname_Model</class>
            </myobserver>
        </models>
    </global>
    <frontend>
        <events>
            <controller_action_predispatch>
                <observers>
                    <mailchimp_ecom_listen>
                        <type>singleton</type>
                        <class>myobserver/observer</class>
                        <method>mailchimpListen</method>
                    </mailchimp_ecom_listen>
                </observers>
            </controller_action_predispatch>
            <sales_order_save_after>
                <observers>
                    <mailchimp_ecom_sale>
                        <type>singleton</type>
                        <class>myobserver/observer</class>
                        <method>mailchimpSale</method>
                    </mailchimp_ecom_sale>
                </observers>
            </sales_order_save_after>
        </events>
    </frontend>
</config>

MailchimpMagentoEcommerce360.php

<?php
class Somecompany_Extensionname_Model_Observer extends Mage_Core_Model_Abstract {

  public function mailchimpListen($observer) {

      $thirty_days = time()+60*60*24*30;

	  	if (isset($_REQUEST['mc_cid']) && isset($_REQUEST['mc_eid'])) {
	        setcookie('mc_campaign_id',trim($_REQUEST['mc_cid']), $thirty_days);
	        setcookie('mc_email_id',trim($_REQUEST['mc_eid']), $thirty_days);
	    }

	}
  
  public function mailchimpSale($observer) {
    
    $order = $observer->getEvent()->getOrder();
    
    if (isset($_COOKIE['mc_campaign_id'],$_COOKIE['mc_email_id'])) {
      $this->addEcomToMailchimp($order);
	  }
    
  }

	//http://apidocs.mailchimp.com/api/rtfm/campaignecommorderadd.func.php

	public function addEcomToMailchimp($order) {

    // include the MCAPI PHP library Mailchimp
    // http://apidocs.mailchimp.com/api/downloads/mailchimp-api-class.zip
		require_once '/full/path/to/mailchimp/MCAPI.class.php';
		require_once '/full/path/to/mailchimp/config.inc.php';

		$MCAPI = new MCAPI($apikey);

		$order_info = array();

		$order_info['id'] = $order->getIncrementId();
		$order_info['campaign_id'] = $_COOKIE['mc_campaign_id'];
		$order_info['email_id'] = $_COOKIE['mc_email_id'];
		$order_info['total'] = $order->getSubtotal();
		$order_info['shipping'] = $order->getShippingAmount();
		$order_info['tax'] = $order->getTaxAmount();
		$order_info['store_id'] = $order->getStoreId();
		$order_info['store_name'] = $order->getStoreName();

		$auxPrice = 0;
		$productsToSkip = array(Mage_Catalog_Model_Product_Type::TYPE_CONFIGURABLE, Mage_Catalog_Model_Product_Type::TYPE_BUNDLE);

		$mcitem = array();

		foreach ($order->getAllItems() as $item) {

			$product = Mage::getSingleton('catalog/product')->load($item->getProductId());

			if(in_array($product->getTypeId(), $productsToSkip) && $product->getPriceType() == 0){

				if($product->getTypeId() == Mage_Catalog_Model_Product_Type::TYPE_CONFIGURABLE){
					$auxPrice = $item->getPrice();
				}

				continue;

			}

      $mcitem['product_id'] = $product->getEntityId();
      $mcitem['product_name'] = $product->getName();

	    $names = array();
	    $cat_ids = $product->getCategoryIds();

			if(is_array($cat_ids) && count($cat_ids)>0){

				$category = Mage::getModel('catalog/category')->load($cat_ids[0]);
				$mcitem['category_id'] = $cat_ids[0];
				$names[] = $category->getName();

				while ($category->getParentId() && $category->getParentId()!=1){
					$category = Mage::getModel('catalog/category')->load($category->getParentId());
					$names[] = $category->getName();
				}
			}

			$mcitem['category_name'] = (count($names))? implode(" - ",array_reverse($names)) : 'None';
			$mcitem['qty'] = $item->getQtyOrdered();
			$mcitem['cost'] = ($auxPrice > 0)? $auxPrice : $item->getPrice();
			
			$order_info['items'][] = $mcitem;
			$auxPrice = 0;
		
		} //end foreach


		try {

			$MCAPI->campaignEcommOrderAdd($order_info);

		} catch (Exception $e) {

			Mage::log($e);

		}

	}

}
?>