<table>
 <tr>
  <td>
    <select name="foo[0][1]"><option value="1">1</option><option value="2">2</option></select>
  </td>
  <td>
    <input type="text" name="text[0][1]" value=""/>
  </td>
 </tr>
</table>

$(function() {
	$("select[name^='foo']").change(function() {
		var ids = /\[\d\]\[\d\]/g.exec(this.name);
		var cmpId = $(this).val();
		$.ajax({
			type: "get",
			url: "index.php",
			dataType: "json",
			data: { ajax: true, id: cmpId },
			success: function(e) {
				// Kein Text drin
				if (e.description === "") alert("Keine Beschreibung vorhanden.");
				// Query Fehler
				else if (e.success === false) alert("Fehler:\n"+e.error);
				// Alles klappt
				else {
					$('input[name="text'+ids+'"]').val(e.description);
				}
			}
		});
	});
});