default controller route for mobile

My site that uses a mobile subfolder in my controllers and views directories to serve up a mobile friendly version when a user logs in from a phone. The login controller makes that determination when the user logs in, and utilizes the appropriate controllers and view. That works fine. However, when a user navigates away from the site and then attempts to hit mysite.com again, the $route['default_controller'] sends them to 'home', which is the normal version of the site. Is it possible to have a conditional statement in the routes.php file that checks if the user is on mobile and then sets the default_controller, or is there a better way to approach this problem? Thanks!