Here is an example  of how to access joomla session data from another php script.
This is particular useful for implementing an AMF gateway or other ajax functionality.

To access the Joomla! session variables from another script, the easiest way is to instantiate the joomla application in that script. Please note that this will only work if that script is not already using its own session.

Example Code:

define( '_JEXEC', 1 );
define('JPATH_BASE', dirname(__FILE__) );

require_once ( JPATH_BASE .DS.'includes'.DS.'defines.php' );
require_once ( JPATH_BASE .DS.'includes'.DS.'framework.php' );

$mainframe =& JFactory::getApplication('site');

$session =& JFactory::getSession();
$user = $session->get( 'user' );

In this example I’m assuming the script is located in the main joomla directory.
If the script sits in some subdirectory you’ll have to adjust the JPATH_BASE definition.