Eliminating Required Shipping Fields for Digital Download Purchases

I’d like WooCommerce to recognize whether or not my customer has placed any physical products in his or her cart. If there are no physical products (only digital downloads), I’d like to deactivate the mailing address requirement. And if they have physical products (like books or DVDs), I want the shipping fields to be required.

Anyone have any experience with this?

Thanks!
–Art

http://wordpress.org/extend/plugins/woocommerce/