Pour rediriger un mobile vers une adresse spécifique il y a plusieurs méthodes :

En  modifiant le htaccess

    RewriteEngine on
    #redirection par htaccess
    RewriteCond %{HTTP_USER_AGENT} ^.*(iPhone|Ipod|IPad|BlackBerry|Palm).*$
    RewriteRule ^(.*)$ http://mobile.site.com [R=301]

Une version plus complète :

RewriteEngine On
RewriteCond %{REQUEST_URI} !^http://m.monsite.fr/.*$
RewriteCond %{HTTP_ACCEPT} "text/vnd.wap.wml|application/vnd.wap.xhtml+xml" [NC,OR]
RewriteCond %{HTTP_USER_AGENT} "acs|alav|alca|amoi|audi|aste|avan|benq|bird|blac|blaz|brew|cell|cldc|cmd-" [NC,OR]
RewriteCond %{HTTP_USER_AGENT} "dang|doco|eric|hipt|inno|ipaq|java|jigs|kddi|keji|leno|lg-c|lg-d|lg-g|lge-" [NC,OR]
RewriteCond %{HTTP_USER_AGENT}  "maui|maxo|midp|mits|mmef|mobi|mot-|moto|mwbp|nec-|newt|noki|opwv" [NC,OR]
RewriteCond %{HTTP_USER_AGENT} "palm|pana|pant|pdxg|phil|play|pluc|port|prox|qtek|qwap|sage|sams|sany" [NC,OR]
RewriteCond %{HTTP_USER_AGENT} "sch-|sec-|send|seri|sgh-|shar|sie-|siem|smal|smar|sony|sph-|symb|t-mo" [NC,OR]
RewriteCond %{HTTP_USER_AGENT} "teli|tim-|tosh|tsm-|upg1|upsi|vk-v|voda|w3cs|wap-|wapa|wapi" [NC,OR]
RewriteCond %{HTTP_USER_AGENT} "wapp|wapr|webc|winw|winw|xda|xda-" [NC,OR]
RewriteCond %{HTTP_USER_AGENT} "up.browser|up.link|windowssce|iemobile|mini|mmp" [NC,OR]
RewriteCond %{HTTP_USER_AGENT} "symbian|midp|wap|phone|pocket|mobile|pda|psp" [NC]
RewriteCond %{HTTP_USER_AGENT} !macintosh [NC] #*SEE NOTE BELOW
RewriteRule ^(.*)$  http://m.monsite.fr/  [L,R=302]

On peut aussi faire une redirection PHP

$user_agent = $_SERVER['HTTP_USER_AGENT'];
if(preg_match("(iPhone|BlackBerry|Android|HTC|LG|MOT|Nokia|Palm|SAMSUNG|SonyEricsson)",$user_agent))
{
     header("Location:http://mobile.monsite.fr"); // redirection vers l'url pour mobile
}