I am using this to replace an existing drupal message to my custom message.
function _exclude_message($mymessage, $mytype="status"){
$flag = false if ($messageArr = drupal_set_message()) { foreach($messageArr as $type=>$messages) {
if ($mytype==$type) {
foreach($messages as $key=>$message) {
if ($mymessage==$message) {
unset($_SESSION['messages'][$type][intval($key)]);
$flag = true;
}
}
}
if (count($_SESSION['messages'][$type])==0) unset($_SESSION['messages'][$type]);}} return $flag;}
function my_theme_status_messages($display = NULL) {
if (_exclude_message('Sorry, unrecognized username or password. <a href="/user/password">Have you forgotten your password?</a>',"error")) drupal_set_message("Sorry, unrecognized username or password.","error");
return theme_status_messages($display);
}
under template.php
this code will replace existing "Sorry, unrecognized username or password. " with my custom message "Have you forgotten your password?".
Hope this will help you.