Jini and J2ME are quite different.
J2ME is a specification for an implementation of
Java for a certain class of machines. It indicates what facilities and APIs should be provided by the JVM and associated APIs. If you have some software which runs on J2ME, the it should work on any J2ME-compliant Java implementation.
Jini is a specification and a set of tools to allow systems (in a loose sense) to associate and collaborate in a robust, dynamic way withour requiring continual system administration and maintenance.
As it stands at the moment, each service in a Jini community needs to be running in a Java Virtual Machine, so Jini can be viewed as a sort of dynamic, self-healing collaboration framework for Java software writers - a layer "above" any particular Java implementation - be that J2ME (*), J2SE,
J2EE or the old "Personal Java".
(*) Last I heard, though, J2ME doesn't actually support enough RMI for Jini. I'd love it to be different - so if anyone knows better, please let us know.