<form action="<?php echo $this->getAddToCartUrl($_product) ?>" method="post" id="product_addtocart_form_<?php echo $_product->getId(); ?>">
<input name="qty" type="text" class="input-text qty" id="qty" maxlength="12" value="<?php echo $this->getMinimalQty($_product) ?>" />
<button class=form-button" onclick="productAddToCartForm_<?php echo $_product->getId(); ?>.submit()"><span><?php echo $this->__('Add to Cart') ?></span></button>
</form>
<script type="text/javascript">
     var productAddToCartForm_<?php echo $_product->getId(); ?> = new VarienForm('product_addtocart_form_<?php echo $_product->getId(); ?>');
     productAddToCartForm_<?php echo $_product->getId(); ?>.submit = function(){
     if (this.validator.validate()) {
             this.form.submit();
         }
     }.bind(productAddToCartForm_<?php echo $_product->getId(); ?>);
</script>