Código PHP:
<?
$sql = mysql_query("select (max(campo_id) + 1)as id from tabla", $conexion);
$rs = mysql_fetch_array($sql);
$idnuevo=$rs["id"];
?>
<html>
<body>
<form name="frm" .......>
<input type="text" id="idnuevo">
<input type="button" value="nuevo" onClick="javascript: document.getElementById('idnuevo').value = <?=$idnuevo?>;">
</form>
</body>
<html>