<?php

class Custom_Adminhtml_Block_Catalog_Product_Grid extends Mage_Adminhtml_Block_Catalog_Product_Grid
{
    protected function _prepareColumns()
    {
        $this->addColumn('preview', array(
            'header' => Mage::helper('catalog')->__('Preview'),
            'width' => '50px',
            'type' => 'action',
            'actions' => array(
                'caption' => Mage::helper('catalog')->__('Preview'),
                'url' => array(
                    'base' => '/catalog/product/view'
                    'params' => array()
                ),
                'field' => 'id'
            ),
            'filter' => false,
            'sortable' => false,
            'index' => 'id'
        ));

        return parent::_prepareColumns();
    }
}