If you need SSO between web applications deployed to the same HOST, then you can use the Apache Tomcat SingleSignOnValve. If you need to do SSO across a JBoss Cluster, then you will need the ClusteredSingleSignOnValve. Take a look at the following clustered single sign on white paper here.
More details are here: Single Sign On in JBoss
Additional reference: http://anil-identity.blogspot.com/2009/02/jboss-identity-community-platform.html