The following HTML does this. It uses the onchange event of the option select box and a javascript function.
I posted it on my website if you want to see it working:
http://adam.codedv.com/textbox.html
Code:
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Textbox Transfer</title>
<script type="text/javascript">
<!--
function processchange ()
{
var theForm = document.getElementById('questions');
theForm.answertext.value = theForm.answers.value;
}
//-->
</script>
</head>
<body>
<form action="submit.php" id="questions">
<p>
<input type="text" name="answertext" />
<select name="answers" onchange="processchange();">
<option>answer1</option>
<option>answer2</option>
<option>answer3</option>
</select>
</p>
</form>
</body>
</html>