Jquery :: Only Validate Textbox If Corresponding Checkbox Checked

I have a simple form which I am running a validation on a single text box (as shown below, adapted from the milk example).

$(document).ready(function() {
// validate signup form on keyup and submit
var validator = $("#updateModules").validate({
rules: {
guestbookContact: {
required: true,
email: true
},
},
// the errorPlacement has to take the table layout into account
errorPlacement: function(error, element) {
if ( element.is(":radio") )
error.appendTo( element.parent().next().next() );
else if ( element.is(":checkbox") )
error.appendTo ( element.next() );
else
error.appendTo( element.parent().next() );
},
// specifying a submitHandler prevents the default submit, good for the demo
submitHandler: function() {
form.submit();
},
// set this class to error-labels to indicate valid fields
success: function(label) {
// set   as text for IE
label.html(" ").addClass("checked");
}});

Now I only want to validate the text box if the checkbox "requireEmail" is checked. Is there an easy way to do this?