prettyremage,
Вот так будет работать:
<form>
<input type="text" size="30" name="txt_field" id="txt_field">
<input type="button" name="button" value="ввести текст" ONCLICK="insert_text(document.getElementById('txt_field').value)">
<div id="result"></div>
</form>
<script type="text/javascript">
function insert_text(text)
{
document.getElementById('result').innerHTML=text;
}</script>