Redirect to mobile page (not theme switching)
I suppose if you really wanted to, you could do something along the lines of this in a custom module:

function YOURMODULE_init() {

  if ($_SERVER['REQUEST_METHOD']=='GET' // most ajax/ahah etc use POST
      &amp;&amp; arg(0)!='admin' // admin theme/backend
      &amp;&amp; strpos($_GET['q'], 'sites/default/files')===FALSE // are we looking at anything under files that doesn't "really" exist, eg imagecaches that aren't created yet
      &amp;&amp; $_GET['q']!='m') { // aren't already "here"

    include 'PATHTOSCRIPT/Mobile_Detect.php'; 
    $detect = new Mobile_Detect();

    if ($detect-&gt;isMobile()) {
      drupal_goto('m');
    }

  }

}


but they could never leave, or rather, be able to go to any other page on your site. 

Addition:  per @MPD's comments, some further protections.