How would you access SAP from a
Servlet or a
JSP? You can do it pretty much the same way from a portlet. Remember, a portlet is just a rendering tool, in its most basic sense. Connecting to back ends shouldn't be any different from a portlet than a Servlet.
Of course, service out as much as possible. If you can provide access to SAP through a web service, and then connect to the web service through the portlet, that's a bit more service oriented, and better than connecting directly from the portlet.
Of course, if your question is how to connect to SAP from a Servlet/JSP app as well, then, I guess my answer really isn't much help to you.
-Cameron McKenzie