Mobile is based on user agent. Meaning what the browsers returns back.
I have these defined. What device are you using?
$user_agents = array(
array('iPhone', 'iphone'),
array('iPod', 'ipod'),
array('iPad', 'ipad'),
array('PocketIE', 'iemobile'),
array('Opera Mini', isset($_SERVER['HTTP_X_OPERAMINI_PHONE_UA']) ? 'operamini' : 'operamini'),
array('Opera Mobile', 'Opera Mobi'),
array('Android', 'android'),
array('Symbian', 'symbian'),
array('BlackBerry', 'blackberry'),
array('BlackBerry Storm', 'blackberry05'),
array('Palm', 'palm'),
array('Web OS', 'webos'),
);