In you weblogic deployed application when accessing the WebLogic MBeans, you should be aware that BEA changed the default value for the AnonymousAdminLookupEnabled attribute in SP5 of WebLogic 8.1.
Prior to SP5, the default was true, allowing lookup of the MBeans in JNDI with the anonymous user. From SP5 and ahead, the default is false.
Either put a login in the InitialContext or edit your config.xml to read:
Currently, the docs are still documenting it to have a default of true.