<%= submit_tag 'Submit', :onclick => 'return validate(this)' %><br/>

<script type="text/javascript" charset="utf-8">
  function validate() {
    var valid = false;
    var radio_groups = {}
    $(":radio").each(function(){
      radio_groups[this.name] = true;
    })
    for(group in radio_groups){
      if (!!$(":radio[name="+group+"]:checked").length) {
          valid = true;
      }
    }
    if (!valid) {
      alert('you forgot to check a radio button somewhere');
      return false;
    }
  }
</script>