I'm going to document my progress on getting JBoss Portal 2.6 working with JBoss Seam to develop simple portlets. Right now, I've gotten JBoss Portal 2.6.DR1 running in JBoss AS 4.0.5.GA with an EJB 3 configuration installed.
All I had to do so far was run the JBoss AS installer (I did it via webstart), and select an EJB 3 configuration. Then I downloaded the JBoss Portal 2.6.DR1 binaries (not the bundle), created a "portal" directory under the deploy directory in my EJB 3 configuration in JBoss AS, then copied in the jboss-portal.sar, and the portal-hsqldb-ds.xml file (from the "setup" directory in the JBoss Portal 2.6.DR1 archive).
So far, everything boots, and I can access the portal. Now to try my hand at creating a Seam Portlet.