Sys.Mvc.ValidatorRegistry.validators["accept"] = function (rule) {
    // initialization code can go here.
    var accepted = rule.ValidationParameters["accepted"];

    // we return the function that actually does the validation 
    return function (value, context) {
        if (context.fieldContext.elements[0].checked == accepted) {
            return true;
        }
        return rule.ErrorMessage;
    };
};