[Jon]

Should be something like this. Open cart.php

Find:

# Assign the current location line
$smarty->assign("location", $location);

Before add:

// WCM - Identify product
// Change X to your numeric productid
if (is_array($cart) AND !empty($cart['products'])
    foreach($cart['products'] as $k=>$v)
        if ($v['productid'] == X)
            $smarty->assign('found_product', 1);
// / WCM - Identify product

In your .tpl file of checkout use:

{* WCM - Identify product *}
{if $found_product eq 1}
Display my message
{/if}
{* / WCM - Identify product *}