When I did it in my cart, the button did not say, "Ask a Question" (I didn't use the supplied button), so I had to add some link text - but I forgot to delete this when I posted the solution. Thanks to Phid for noticing this - but he has missed one problem line (the extra div). The correct code is

<div id="askQuestion" class="biggerText">
<?php
echo '<a href="' . zen_href_link(FILENAME_ASK_A_QUESTION, 'products_id=' . $_GET['products_id']) . '">' .
zen_image_button(BUTTON_IMAGE_ASK_A_QUESTION, BUTTON_ASK_A_QUESTION_ALT) . '</a>'; ?>
</div>
<br />
<br class="clearBoth" />

Thanks,
Scott