Validar un grupo de checkboxes exigiendo que esté marcado más de uno From http://stackoverflow.com/questions/2445010/jquery-validation-plugin-validating-checkboxes-with-different-names

validation.js

$.validator.addMethod('require-one', function (value) {
              return $('.require-one:checked').size() > 0; }, 'Please check at least one box.');

var checkboxes = $('.require-one');
var checkbox_names = $.map(checkboxes, function(e,i) { return $(e).attr("name")}).join(" ");

$("#itemForm").validate({
    groups: { checks: checkbox_names },
    errorPlacement: function(error, element) {
             if (element.attr("type") == "checkbox")
               error.insertAfter(checkboxes.last());
             else
               error.insertAfter(element);
    }         
});