Get permalink for a post from inside WordPress and route to a related site

I’d use responsive design rather than a mobile subdomain copy of the site, however if you really do want this, then put this code in a plugin:

// this function doesn't exist prior to wordpress v3.4
if(!function_exists('wp_is_mobile')){
   /**
    * Test if the current browser runs on a mobile device (smart phone, tablet, etc.)
    *
    * @return bool true|false
    */
   function wp_is_mobile() {
       static $is_mobile;

       if ( isset($is_mobile) )
           return $is_mobile;

       if ( empty($_SERVER['HTTP_USER_AGENT']) ) {
           $is_mobile = false;
       } elseif ( strpos($_SERVER['HTTP_USER_AGENT'], 'Mobile') !== false // many mobile devices (all iPhone, iPad, etc.)
           || strpos($_SERVER['HTTP_USER_AGENT'], 'Android') !== false
           || strpos($_SERVER['HTTP_USER_AGENT'], 'BlackBerry') !== false
           || strpos($_SERVER['HTTP_USER_AGENT'], 'Opera Mini') !== false ) {
               $is_mobile = true;
       } else {
           $is_mobile = false;
       }

       return $is_mobile;
   }
}
add_action('init','mobile_redirect_init');
function mobile_redirect_init(){
    if(wp_is_mobile()){
        $url="http://m.".$_SERVER["HTTP_HOST"].$_SERVER["REQUEST_URI"];;
        wp_redirect($url);
    }
}