[jQuery] EqualTo but only if checkbox is checked

[[EMAIL PROTECTED]]

How can I compare 2 text fields only if a certain checkbox is
checked?  Can you do something like this?

billingAddress: {
        required: true
        minlength: 5
        equalTo: "#shippingAddress" && function(element) {
                return jQuery("#comparefields").is(":checked");
},

I'm not sure if this is possible or how to form this correctly.  I
assume you'd be able to do this in an equalto along with specifying
which field to compare to.