Con Javascript puedes hacer algo asi:
Código:
<select name="valores" onchange="populateText(this);">
<option value="1">Uno</option>
<option value="2">Dos</option>
<option value="3">Tres</option>
</select>
<input type="text" name="valorDesc" id="valorDesc" value="" />
<script type="text/javascript">
function populateText(selObj) {
var desc = document.getElementById( "valorDesc" );
var texto = selObj.options[selObj.selectedIndex].text;
desc.value = texto;
}
</script>