IMO, if you are running under JBoss, then MBean makes sense. Yes, they will share the same cache instance.
If you are _not_ deploying the cache as MBean but instantiate it from your own code (using the constructor and PropertyConfigurator) each instance will connect to the same Group sharing the same data, but will not be the same cache instance. (As a consequence the data would reside in memory multiple times).
To use a single cache-instance (without using the MBean deploment) you have to write your own factory-class implementing a static factory-method that allways returns a reference to the same TreeCache-instance. (Or instantiate a single TreeCache-instance from some startup-class that binds the TreeCache to JNDI so every session-bean might retrive the same instance (in a single VM) from there.
Thanks for the replies. This helps make a decision. In the case of using JNDI, is the JNDI code in JBossCache (that I keep reading about) working at the moment? Thanks again.