echo in ordine inverso da mysql_fetch_assoc()

Archivia i tuoi dati e usa array_reverse ,

while($row = mysql_fetch_assoc($result)){
    $items[] = $row;
}

$items = array_reverse($items ,true);

foreach($items as $item){
   echo $item['time']." - ".$item['username']." - ".$item['message'];
}