This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
For a very few special packages -- the org.omg CORBA packages, for instance -- you can override the version that comes with your JRE by dropping the jars into an "endorsed" directory. These are packages that implement "endorsed standards" -- basically things that get shipped with the JDK but that are governed by some third-party standards body. Anyway, this only works for packages on this short list, and in general, most jars in such a directory are just going to be ignored.