<tr>
      <td><label for="subject">Тема</label></td>
      <td>
          <select name="subject" id="subject">
            <option value="cartridge">Тема1</option>
            <option value="application">Тема2</option>
          </select>
      </td>
      <td> </td>
    </tr>
      <tr>
        <td><label for="number">Текст1</label></td>
        <td><input type="text" style="width:200px;" class="validate[required]" name="number" id="number" maxlength="100" value="" />
    </td>
      </tr>
    <tr>

    <tr>
<script>
function changeText(select) {
  s = $("#subject").attr("value");
  if(s == "cartridge") t = "Номер";
  if(s == "application") t = "Срок";
  $("#numberLabel").text(t);
}
</script>
      <td><label for="subject">Тема</label></td>
      <td>
          <select name="subject" id="subject" onchange="changeText(this)">
            <option value="cartridge">Закончился картридж</option>
            <option value="application">Произвольная заявка</option>
          </select>
      </td>
      <td>&nbsp;</td>
    </tr>
      <tr>

        <td><label for="number" id="numberLabel">Номер</label></td>

        <td><input type="text" style="width:200px;" class="validate[required]" name="number" id="number" maxlength="100" value="<?=$post_number?>" />&nbsp;<img src="images/icon.gif" id="number-info" width="14" height="14" alt="Подробней" />
    <script type="text/javascript">$('#number-info').tooltip({delay:0, showURL:false, opacity:1.0, bodyHandler: function() { return $("<img/>").attr("src", "images/sticker-help.jpg");}});</script>
    </td>
        <td>&nbsp;</td>
      </tr>