Okay, so this thread is way out dated but there was no solution... so I went ahead and threw together a little script. It took me several hours to get it just right.. but it was sitting right in my face the whole time.
It uses the attribute of MINIMUM ORDER QUANTITY, so for every product in your page, all you need to do is change that to get the result you're trying to achieve. I believe it's default at 1.
Solution:
In the file: tpl_product_info_display.php for your template (in public_html/includes/templates/YOURTEMPLATE/templates/tpl_product_info_display.php) find the lines that are surrounded by:
"bof Add to Cart Box" and "eof Add to Cart Box". Then, simply replace with the following code. BACKUP BEFORE YOU CHANGE THIS!!
Code:
<!--bof Add to Cart Box -->
<?php
if (CUSTOMERS_APPROVAL == 3 and TEXT_LOGIN_FOR_PRICE_BUTTON_REPLACE_SHOWROOM == '') {
// do nothing
} else {
?>
<?php
$optvalues = '';
for ($i = 1; $i < 76; $i++) {
$optvalues .= '<option value="' . ((zen_get_buy_now_qty($_GET['products_id'])) * $i) . '">'. ((zen_get_buy_now_qty($_GET['products_id'])) * $i) .'</option>';
}
$display_qty = (($flag_show_product_info_in_cart_qty == 1 and $_SESSION['cart']->in_cart($_GET['products_id'])) ? '<p>' . PRODUCTS_ORDER_QTY_TEXT_IN_CART . $_SESSION['cart']->get_quantity($_GET['products_id']) . '</p>' : '');
if ($products_qty_box_status == 0 or $products_quantity_order_max== 1) {
// hide the quantity box and default to 1
$the_button = '<input type="hidden" name="cart_quantity" value="1" />' . zen_draw_hidden_field('products_id', (int)$_GET['products_id']) . zen_image_submit(BUTTON_IMAGE_IN_CART, BUTTON_IN_CART_ALT);
} else {
// show the quantity box
$the_button = PRODUCTS_ORDER_QTY_TEXT . '<select name="cart_quantity">'. $optvalues .'</select><br />' . zen_get_products_quantity_min_units_display((int)$_GET['products_id']) . '<br />' . zen_draw_hidden_field('products_id', (int)$_GET['products_id']) . zen_image_submit(BUTTON_IMAGE_IN_CART, BUTTON_IN_CART_ALT);
}
$display_button = zen_get_buy_now_button($_GET['products_id'], $the_button);
?>
<?php if ($display_qty != '' or $display_button != '') { ?>
<div id="cartAdd">
<?php
echo $display_qty;
echo $display_button;
?>
</div>
<?php } // display qty and button ?>
<?php } // CUSTOMERS_APPROVAL == 3 ?>
<!--eof Add to Cart Box-->
And you're done. This is my first contribution to the open source community. 
Tommy
http://www.newcorporateimage.com