Well, to be more truthful a prototype is working on AS5 :) This is more than just a simple update to get Portal running on top of AS5, we are working to take advantage of the JBoss Microcontainer.
The biggest change so far is with deployment. Portlets are now being deployed with a special portal deployer which facilitates the deployment with Portal runtime (formally the jboss-portal.sar). The runtime's structure has even changed from a nested deployment inside the sar to individual deployments.
For everybody out there who can't wait to give this a try, see http://www.jboss.org/community/docs/DOC-13435. There might be a few issues remaining, so please bring them up in the user forum or open a bug.
Stay tuned, more updates to come as we move closer to a release.