Depends a lot on what mode you're using (standard checkout or anonymous checkout).
To save changing more than 1 section of code, a good place to put this code is in cart_details.tpl (skin1/customer/main/cart_details.tpl}.
Look for the section of code like this
Code:
{section name=prod_num loop=$products}
<tr>
<td class=ProductPriceSmall>{if $config.Appearance.allow_update_quantity_in_cart eq "N" or ($active_modules.Egoods and $products[prod_num].distribution) or ($active_modules.Subscriptions and $products[prod_num].sub_plan)}{$products[prod_num].amount}{else}<input type=text size=3 name="productindexes[{$products[prod_num].cartid}]" value="{$products[prod_num].amount}">{/if}</td>
<td>{$products[prod_num].productcode}</td>
<td>{$products[prod_num].product|truncate:30:"...":true}</td>
<td class=ProductPriceSmall align=right>{include file="currency.tpl" value=$products[prod_num].price}</td>
<td class=ProductPriceSmall align=right>{include file="currency.tpl" value=$products[prod_num].total}</td>
</tr>
{/section}
Just before the {/section}
add these lines
Code:
{if $products[prod_num].productid eq "10" and $checkout_step eq "1" }
<tr><td align="center" colspan="{$colspan}">
<font color="red">This only appears when product 10 is selected</font></p></td></tr>
{/if}
Depeding on how you've based your products, you may have to change productid to productcode.
You can change the message appearance to whatver you want with CSS or font ..oh!, can also change the message!
LeeD