If you try to run Portal 2.6.5.SP1 on JBossAS 4.2.3, you will get a lot of errors. The reason is that the "default" configuration of JBossAS no longer contains the JBoss Cache JAR file in the lib directory. You can copy the JAR file from the "all" config, and Portal is then happy:
copy server/all/lib/jboss-cache-jdk50.jar => server/default/lib/jboss-cache-jdk50.jar
Disclaimer: haven't played with it too much yet, so there could be other things lurking. Will know more in a few hours.
2.6.6 will be the first version to be tested on AS 4.2.3.
It's already tagged and starting its QA phase.
(JBoss Cache has been added to the CMS)