Thank's _ck_ !
Ok here is my final code and it work !
1) create a costum page in wordpress with costum template. exemple : create a blank page with url : http://www.monsite.com/thanks
use this template :
<?php
/*
Template Name: thank's
*/
?>
<!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" dir="ltr" lang="fr-FR">
<head profile="http://gmpg.org/xfn/11">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<meta http-equiv="refresh" content="3;url=<?php echo $_GET["re"]; ?>">
<title>Merci !</title>
</head>
<body>
<div style="text-align:center">
Merci de vous être connecté !
<p>Vous allez être rediriger automatiquement vers la page d'ou vous venez ...</p>
<p>Si votre navigateur ne vous redirige pas automatiquement ">cliquez ici.</p>
</div>
</body>
</html>
2) create a template called "my_login.php" with the following code and upload it on your wordpress theme :
<form class="login" method="post" action="http://www.monsite.com/forum/bb-login.php">
<label>Username: <input name="user_login" type="text" id="user_login" size="13" maxlength="40" value="" tabindex="1" />
</label>
<label>Password: <input name="password" type="password" id="password" size="13" maxlength="40" tabindex="2" />
</label>
<input name="re" type="hidden" value="http://www.monsite.com/thanks?re=<?php echo get_permalink(); ?>" />
<input type="submit" name="Submit" id="submit" value="Log in »" tabindex="4" />
<input name="remember" type="hidden" id="remember" value="1" tabindex="3" checked />
</form>
finaly call this template on your header.php like :
<?php
function wt_get_user_name()
{
global $userdata;
get_currentuserinfo();
return $userdata->user_login;
}
if ( is_user_logged_in() ) {
echo 'Welcome ' . wt_get_user_name();
} else {
include (TEMPLATEPATH . '/my_login.php');
};
?>
All done 
