[PhilJ]

Many thanks Adian009

if (product_option('Product Option').value == '')
{ alert('You must enter an option'); 
return false; 
}
else {return true;}

You can still bypass it, by pressing Enter in the product option field, so I disabled the Enter key...

Add this script to the top of skin1/customer/main/product.tpl

{literal}
<script type="text/javascript">
function handleEnter (field, event) {
		var keyCode = event.keyCode ? event.keyCode : event.which ? event.which : event.charCode;
		if (keyCode == 13) {
			var i;
			for (i = 0; i < field.form.elements.length; i++)
				if (field == field.form.elements[i])
					break;
			i = (i + 1) % field.form.elements.length;
			field.form.elements[i].focus();
			return false;
		} 
		else
		return true;
	}      

</script>
{/literal}

then find
/skin1/modules/Product_Options/customer_options.tpl

change

<input type=text name="product_options[{$product_options[product_option].optclass}]" size=12>

to

<input type=text name="product_options[{$product_options[product_option].optclass}]" onkeypress="return handleEnter(this, event)" size=12>