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.
It may get overwritten whenever the JVM is updated, which is a rare occasion - in contrast to Sun, Apple releases new JVM versions no more than once a year or so (more likely less often than more often).
And there's no way to install multiple versions of Apple's releases.
If you're talking about Harmony, OpenJDK/IcedTea, Kaffe, SableVM or other such 3rd-party JVMs - that's possible, but those wouldn't be installed in the same location as Apple's JVM, so no harm done.