We use JBoss 5.1.0.
A standar war WEB-INF/lib/*.jar are not accessable outside of that WAR application.
The WAR webapp can access class in your their own WEB-INF/lib/*.jar obviously, and if it not found there, it then looks for those in JBOSS_HOME/server/<profile-server>/lib, then JBOSS_HOME/common/lib (if you use default), and then JBOSS_HOME/lib. But applications outside of the webapp in JBoss can't access the war's WEB-INF/lib/*.jar.
A webapp WAR's WEB-INF/lib/*.jar are automatically reloaded whenever you redeploy your war file. All other server lib directories requires you to restart JBoss to pickup new jars.
Search JBoss classloader in wiki and google to see more info.