This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
All versions of OS X have Java installed. Depending on which version of OS X is used, one or more JVMs will be available (1.3, 1.4, 5, 6). Running /Applications/Utilities/Java Preferences.app shows what's available, and allows the default JVM to be set.