Hi,
Remember that Portlets are extentions built on the
Servlet framework. The Portlet API provides PortletRequestDispatcher interface to help delegate the creation of contents to Servlets and JSPs. Please note that the portlet request dispatcher can be used ONLY during render requests. Also, one of the differences between portlet and servlet request dispatcher is that the portlet request dispatcher doesn't have the forward method.
You can find an extremely lucid description on section PLT16 of the Portlet 1.0 Specifications.
Regards,
Sandeep