[Яростный Меч]

самый простой вариант - сделать так:

function insertSmile() {
    var smileId = this._smileId;
    var smileInsertingCode = '[img]http://forum.mcmodding.ru/smilesSystem/images/' + smileId + '.png[/img]';
    var textArea = document.getElementById('theTextArea');
    var _caret = textArea.value.length + smileInsertingCode;
    textArea.value = textArea.value + smileInsertingCode;
    textArea.setSelectionRange(_caret, _caret);
    textArea.focus();
}

....
smilesObjectArray[i].onclick = insertSmile;
smilesObjectArray[i]._smileId = smilesIdArray[i];
....