Currently if I use facebook's require_login, it appears to go into an infinite loop of appending to the url. All of my controllers inherit from a FB_Controller that upon construction does:
$this->facebook = new Facebook($this->API_KEY, $secret);
$this->uid = $this->facebook->require_login(); //Causes infinite loop!
I had to explicitly set the canvas callback URL on Facebook to http://decider.dfgh.org/ask/index/ (where ask is my desired default controller) or else I get a 404 error. Also I'm using the iframe rendering method.
On CodeIgniter I have the typical mod_rewrite to get rid of index.php, $config['uri_protocol'] = "REQUEST_URI", $config['enable_query_strings'] = TRUE. The latter two configurations I had to set to get simple linking to work (clicking on a link like http://decider.dfgh.org/decide/ would not work, it would just navigate to the current page).