For Fishpig WordPress Series - Custom Wordpress Short Code in Magento http://www.demacmedia.com/magento-commerce/custom-wordpress-shortcode-in-magento-with-fishpig/

config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Demac_FishpigExtension>
            <version>0.1.0</version>
        </Demac_FishpigExtension>
    </modules>
    <global>
        <models>
	<fishpigextension>
	     <class>Demac_FishpigExtension_Model</class>
	</fishpigextension>
        </models>
        <helpers>
	<fishpigextension>
	    <class>Demac_FishpigExtension_Helper</class>
	</fishpigextension>
        </helpers>
        <events>
            <wordpress_shortcode_apply>
                <observers>
                    <fishpigextension>
                        <type>singleton</type>
                        <class>Demac_FishpigExtension_Model_Observer</class>
                        <method>applyShortCodes</method>
                    </fishpigextension>
                </observers>
            </wordpress_shortcode_apply>
        </events>
    </global>
</config>
    

GCB.php

<?php
/*
All extended shortcode class are extended from the helper class - Fishpig_Wordpress_Helper_Shortcode_Abstract 

There are two abstract functions you must implement

public function getTag() 

this functions defines [tagname].



protected function _apply(&$content, Fishpig_Wordpress_Model_Post_Abstract $object)

this function is the tag renderer. 

*/
class Demac_FishpigExtension_Helper_Shortcodes_GCB extends Fishpig_Wordpress_Helper_Shortcode_Abstract
{
    const GCB_TABLE = "_gcb";
    /**
     * 'contentblock' is the tagname for GCB in wordpress, so I follows for compatibility.
     */
    public function getTag()
    {
        return 'contentblock';
    }

    protected function _apply(&$content, Fishpig_Wordpress_Model_Post $post)
    {
        if (($shortcodes = $this->_getShortcodes($content)) !== false) {

            /* Add multisite support */
            $dbhelper = Mage::helper('wordpress/database');
            $multihelper = null;
            $blogId = '';
            if(Mage::getStoreConfigFlag('wordpress/mu/enabled')) {
                $multihelper = Mage::helper('wordpressmu');
                $blogId = $multihelper->getBlogId();
            }
            $tbname = Mage::helper('wordpress/database')->getTablePrefix().$blogId.self::GCB_TABLE;
            /* End - Add multisit support */

            $ra = $dbhelper->getReadAdapter();

            foreach($shortcodes as $shortcode) {
                $id = $shortcode->getParams()->getId();

                $select = $ra->select();
                $select->from($tbname);
                
                if(is_numeric($id)) {
                    $select->where('id = ?', intval($id));
                } else {
                    $select->where('custom_id = ?', $id);
                }

                $rc = $ra->fetchAll($select);

                if(!empty($rc)) {
                    $content = str_replace($shortcode->getOpeningTag(), sprintf('<div class="gcd gcd-%s">%s</div>',$id,$rc[0]['value']), $content);
                }
            }
        }
    }
}

Observer.php

<?php
/*
* Hook the new shortcode to the wordpress_shortcode_apply event
*/
class Demac_FishpigExtension_Model_Observer
{
    public function applyShortCodes($objs)
    {
        $content = $objs->getContent()->getContent();
        $obj = $objs->getObject();
        Mage::helper('fishpigextension/shortcodes_GCB')->apply($content,$obj);
        
        $objs->getContent()->setContent($content);
    }
}

Updated for modern FishPig 2016. Hope to release a OpenSource module that supports a subset of the shortcodes ultimate set