You can use Sun Cluster HA for Apache Tomcat in a scalable configuration if it is guaranteed, that the source ip address remains the same during a session context. This guarantee is achieved in example in an intranet.
If the source ip address might change during a session context and a scalable configuration is required, Apache Tomcat needs to be configured with session replication. This can be done by the application using a global file system or a database.
You will get a performance penalty from this approach.
You will get better performance using Tomcats inbound memory session replication for a scalable configuration with changing source ip addresses.
You can achieve the session replication for Apache Tomcat 4.x by following the instructions at http://www.theserverside.com/resources/article.jsp?l=Tomcat.
For Apache Tomcat 5.x, inbound memory session replication as a built-in feature, is on the feature list.
For Apache Tomcat 3.x the inbound memory session replication is not configurable.