How is Jini any different from RMI/CORBA/...?

From Jini.org

Actually Jini only uses RMI, CORBA, or any other distributed programing protocol so this is not a fair comparison.

It is important to understand that these (RMI/CORBA/SOAP/HTTP/...) are all communications protocols, in that they define how: connections are established, requests are sent, how parameters are encoded, responses are read, and so on. Jini, on the other hand, is an architecture for distributed components where communication protocols are a required aspect to achieve that. See (What is Jini?).

Finally Jini is protocol agnostic but is implemented most of the times using RMI. Just remember, Jini USES RMI and that there is no comparison.