The JSR 168 Portlet Specification (PLT.15.4) states that Portlets are supposed to be able to communicate information with other Servlets and JSPs that are part of the same Portlet Application via their session. It appears that this ability is not possible with Portlet Applications running in Pluto.
If you examine the session ID of the session within a Portlet and then examine the session ID within a Servlet that was accessed directly by the user with a URL pointing to that Servlet, you will see that the session IDs do not match, and apparently they should.
For a demonstration of this, please deploy and examine the following Portlet: http://www.unicon.net/download/PortletSessionTest.war
By the way, the motivation for wanting to share information via the session in this way comes from a need to handle file downloads from a Portlet.
For reference, see the recent discussion thread on the Pluto User Mailing List: