This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
JAR files do have some stuff that has to do with versioning. I don't know how CAB file versioning works so I can't tell you if you can do the same things with JAR files. See the JAR File Specification for more information.