Код:
<html>
<head>
<title>Untitled</title>
<script language="javascript">
<!--
function displayData(nRow, nCell)
{
alert(table1.firstChild.childNodes[nRow].childNodes[nCell].childNodes[0].nodeValue);
}
-->
</script>
</head>
<body>
<table id="table1" border="1">
<tbody>
<tr><td>Data1</td><td>Data2</td></tr>
<tr><td>Data3</td><td>Data4</td></tr>
</tbody>
</table>
<form id="form1" name="form1">
<input type="button" id="btn1" name="btn1" value="Click" onclick="displayData(1, 0)" />
</form>
</body>
</html>
PS : DOM - > RTFM