Ok, this is for the jquery validation module and is being used inside of a jquery call like so:

$("#myform").validate({
  rules: {
    contact: {
      required: true,
      email: {
        depends: function(element) {
          return $("#contactform_email").is(":checked");
        }
      }
    }
  }
});
[download]

So, I'm not familiar enough with jQuery to know if adding the double quotes will break things or not. I guess I could try and see what happens.