A ver si te sirve:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
<script>
function ver(e,m){
var t=e.keyCode || e.wich;
if(t==13){
agregar(m);
return false;
}
return true;
}
function agregar(m){
document.getElementById('chat').innerHTML+='<br />'+m;
document.forms[0].textarea.value='';
}
onload=function(){
setInterval(function(){document.getElementById('chat').scrollTop=document.getElementById('chat').scrollHeight},30);
}
</script>
</head>
<body>
<div id="chat" style="width:400px; height:300px; overflow:auto; border:1px solid #000"></div>
<form id="form1" name="form1" method="post" action="">
<textarea name="textarea" cols="60" rows="3" onkeypress="return ver(event,this.value)"></textarea>
</form>
</body>
</html>