This is happening because JBoss, by default, uses a shared repository for all classes loaded. You probably want to separate your webapps into their own classloaders. See this thread: http://www.jboss.org/index.html?module=bb&op=viewtopic&t=43462. You can search the forums for "UseJBossWebLoader" and find tons of postings discussing this issue.
btw this is the development forum; the user forums (http://www.jboss.org/index.html?module=bb&op=main&c=5) are the right place for this topic