I guess, you'd have found a solution by now. If not, make sure u have mentioned the scope of the variable while accessing the session variable in the jsp.
For instance if u have set a session variable with application scope then make sure u retrieve it as renderRequest.getPortletSession().getAttribute("var",PortletSession.APPLICATION_SCOPE); as it returns Null if we do not mention the scope of the variable. Hope it was useful.
i've solved it a different way (APPLICATION_SCOPE works only for portlets inside the same war/ear, and additional i want to access the session from Portal [not a portlet]).
now i have written a ServletReqListener which put the HttpSession from the Portal into a static-ThreadLocal Class-Member. With this one i can access the Portal-HttpSession from each portlet.
Works fine and now i have a <portal:maincontent> tag for the simple-gatein-portal which renders the selected portlet (read from global session).
Thanks to JSF-statefullness i can change portlets like an application-switcher (change back to a jsf-portlet restores last view).
Can you share your solution with me? I am trying to do the same thing...
Let me know so I can give me my private email address for communication..