Odd problem with javascript and smartphones

Hi - I have an odd problem turn up when using some javascript in the checkout to update the shipping costs. I have an overload statement -

FC.checkout.overload("updatePrice", "updateMyShippingCost", null);

to recalculate the shipping costs based on the destination country of the shipment and whether or not there is gift wrapping involved. The function that does the recalculation looks like this -

function updateMyShippingCost() {
	var numProducts = fc_json.product_count;
	var basePrice = euro_uk_base;
	var increment = euro_uk_increment;
	var country_code = (jQuery("#use_different_addresses").is(":checked") ? $("#shipping_country").val() : $("#customer_country").val());
	if (country_code != "GB") {
		basePrice = euro_elsewhere_base;
		increment = euro_elsewhere_increment;		
	}
	shippingCost = basePrice + (numProducts - 1) * increment;
	if (jQuery("#gift_wrap").is(":checked")) { shippingCost = shippingCost + euro_gift_wrap; }
	FC.checkout.config.orderFlatRateShipping = shippingCost;
}

This works perfectly on desktop browsers but doesn't seem to work on either safari or chrome on an iphone. I tried putting in a little alert(country_code) statement and it was always set to GB regardless what was set in either the shipping_country or customer_country fields.

It's hard to see what's going on because it's only on the iphone I've seen this (I haven't tried any other mobile/smartphone devices because I don't have easy access to them). I know it's a slightly obscure problem but has anyone come up against anything similar?

regards,
Gordon