If you go to wsad menu
windows -> preferences -> installed JRE
you can give the location of the other
java / jre installations. If you wanted to use say the latest sun jdk to
test out something you can do that using this.
Also if you are using
tomcat you can specify the jdk to use with that instance of tomcat. For example You can have one jdk to use with websphere, one with tomcat 3.X and one with tomcat 4.X.