[funkydunk]

no probs.

I used:

{if $product.param00 eq ""}

Put the normal add to cart stuff here

{else}
{include file="buttons/order_now.tpl"}
{/if}

I used param00 but use whatever you feel fit.

you will also need to either switch off extra fields and add the param00 update box to the product_modify template in main or amend modules/extra_fields/product.tpl to stop it displaying the url in the product details.