Have you taken a look at the latest clustering doco? there is a full step-by-step procedure on how to setup this scenario. Have you activated clustering in your web.xml file?
My bad -- I had turned it (the distributable flag) off for some tests, and then I forgot to turn it back on. It works like a charm.
Great. And thank you for posting positive feedback as well: we frequently have questions, we give answer and never now if that was successful or not.