Use a block in your grid column renderer

app_code_community_[Namespace]_[ModuleName]_Block_Adminhtml_Item_Grid.php

<?php
/**
 * Namespace ModuleName
 *
 * @category    Namespace
 * @package     Namespace_ModuleName
 * @copyright   Copyright (c) 2015 Tobias Forkel (http://www.tobiasforkel.de)
 * @license     http://opensource.org/licenses/osl-3.0.php Open Software License (OSL 3.0)
 */

class Namespace_ModuleName_Block_Adminhtml_Item_Grid extends Mage_Adminhtml_Block_Widget_Grid
{
    /**
     * Prepare grid columns
     *
     * @return Namespace_ModuleName_Block_Adminhtml_Item_Grid
     */
    protected function _prepareColumns()
    {
        $this->addColumn('is_active',
            array(
                'header'    => $this->__('Active'),
                'index'     => 'is_active',
                'renderer'  => 'Namespace_ModuleName_Block_Adminhtml_Item_Renderer_IsActive'
            )
        );
    }
}

app_code_community_[Namespace]_[ModuleName]_Block_Adminhtml_Item_Renderer_IsActive.php

<?php
/**
 * Namespace ModuleName
 *
 * @category    Namespace
 * @package     Namespace_ModuleName
 * @copyright   Copyright (c) 2015 Tobias Forkel (http://www.tobiasforkel.de)
 * @license     http://opensource.org/licenses/osl-3.0.php Open Software License (OSL 3.0)
 */

class Namespace_ModuleName_Block_Adminhtml_Item_Renderer_IsActive extends Mage_Adminhtml_Block_Widget_Grid_Column_Renderer_Abstract
{
    /**
     * Render
     *
     * @param Varien_Object $row
     *
     * @return string
     */
    public function render(Varien_Object $row)
    {
        return $this->getLayout()->createBlock('namespace/modulename/adminhtml_item_cell_isActive')->setRow($row)->toHtml();
    }
}

app_code_community_Namespace_ModuleName_Block_Adminhtml_Item_Cell_IsActive.php

<?php
/**
 * Namespace ModuleName
 *
 * @category    Namespace
 * @package     Namespace_ModuleName
 * @copyright   Copyright (c) 2015 Tobias Forkel (http://www.tobiasforkel.de)
 * @license     http://opensource.org/licenses/osl-3.0.php Open Software License (OSL 3.0)
 */

class Namespace_ModuleName_Block_Adminhtml_Entity_Cell_IsActive extends Mage_Adminhtml_Block_Widget_Form
{
    protected function _construct()
    {
        parent::_construct();
        $this->setTemplate('namespace/modulename/adminhtml/item/cell/isactive.phtml');
    }
}

app_design_adminhtml_default_default_template_namespace_modulename_adminhtml_item_cell_isactive.phtml

<?php $row = $this->getRow(); ?>

<strong>The content for your grid cell. Do something with $row.</strong>