you cannot upgrade the Infinispan modules used by the internal AS clustering subsystem (that's where that message is coming from). You can however deploy our modules to import them in your application (and it seems you've configured that right). Just deploy an Infinispan-using application and check the version from there.
Ahh ok that makes perfect sense, thanks Tristan
One question then, if Jboss is using 5.2.1 for the clustering subsystem and I was to write a custom Externalizer for some non-serializable classes in my application should I refer to the 5.x docs for the Externalizer?