Starting JBoss-5.0 GA version, you will notice that the %JBOSS_HOME%/server/default/lib older is empty (and the other %JBOSS_HOME%/server/< serverName>/lib folders have very few jar files).
This was an intentional change in the GA version of JBoss-5. The lib folder under each server profile (default, all etc...) is now empty and all the jar files are now moved to a %JBOSS_HOME%/common/lib folder. This common/lib will act as a parent to all server/< serverName>/lib folders. If you want to add any jar for a specific server profile, then you can add it to the %JBOSS_HOME%/server/< serverName>/lib folder.