I've been toying with the Facebook Platform the the past few days and am impressed by it thus far, minus the lack of documentation and usage examples. I created a base class to extend applications from in order to get a feel for how application are structured. It is based after the only coherent piece of documentation I have been able to find, here. I will publish my code soon; in the mean time, here is an example:

class FacebookApplication {
...
     function get_friends_with_app_installed() {
             $fql = "SELECT uid from user where uid in (SELECT uid2 FROM friend WHERE uid1=".$this->get_client_id().") and is_app_user=1;";
             return $this->get_client()->api_client->fql_query( $fql );
     }
}

class LookMiiUpFacebookApp extends FacebookApplication {
...
   function splash() {
           echo "lookmiiup splash";
           print_r( $this->get_friends_with_app_installed() );
   }

   function set_info() {
           $wii = strip_tags($_REQUEST["wii"]);

           $facebook = $this->get_client();
           $user = $this->get_client_id();

           $facebook->api_client->profile_setFBML("Wii Number: $wii", $user);
           $facebook->api_client->feed_publishActionOfUser( "got a new Wii number", "$wi
i" );

           echo "wii: $wii";
   }

   function handle_request() {
           static $allowed_methods = array(
                   "set_info"
           );

           if( in_array( $method, $allowed_methods ) ) {
                   $params = array();
                   call_user_func_array( array( $this, $method ), $params );
           } else {
                   parent::handle_request();
           }
   }

}

$fbapp = new LookMiiUpFacebookApp( $appapikey, $appsecret, $appcallbackurl );

$fbapp->require_login();
$fbapp->require_installation();

$fbapp->handle_request();

The added value is the ability to more intuitively (in my opinion, anyway) create an application that requires login/installation and setup the handler methods. I plan to use it, with Smarty for FBML generation, to integrate LookMiiUp into Facebook.