Mobile version and URL Rewrite

You can change the options siteurl and home.

add_action('after_setup_theme', 'maybe_mobile_url');
function maybe_mobile_url() {
  if ( wp_is_mobile() ) {
    add_filter('option_siteurl', 'add_mobile_url' );
    add_filter('option_home', 'add_mobile_url' );
  }
}
function add_mobile_url ($url) { return $url . '/mobile'; }

But, I think, that if you define WP_SITEURL and/or WP_HOME in config.php it will fail, and you will have to add filter to all the function that retrieve permalinks.