<form id="form1" name="form1" method="post" action="">
  <table border="1">
    <tr>
      <td width="56">Nome:</td>
      <td width="144"><label>
        <input type="text" name="nome" id="nome" />
      </label></td>
    </tr>
    <tr>
      <td>Telefone:</td>
      <td><label>
        <input type="text" name="telefone" id="telefone" />
      </label></td>
    </tr>
    <tr>
      <td>Voce tem Celular?</td>
      <td><input type="checkbox" id="sim"> SIM</a>
<input type="checkbox" id="nao"> não</a>
<script>
    $("#sim").click(function() {
           if($(this).prop("checked")) {
           $("#teste").attr("style","display:block;");
         }
    else {
     $("#teste").attr("style","display:none;");
    }
        });
</script>

<input type="text"id="teste"style="display:none;">
</td>
    </tr>
    <tr>
      <td>Email:</td>
      <td><label>
        <input type="text" name="email" id="email" />
      </label></td>
    </tr>
    <tr>
      <td>&nbsp;</td>
      <td>&nbsp;</td>
    </tr>
  </table>
</form>