Sun believes that the new release of Java is so revolutionary that a new number is required, so we are going from Java 1.4 to Java 5.0. Have the engineers at Sun suddenly forgotten how to count?
If you think about it, the transition to Java 1.2 was much more revolutionary than this transition but Sun didn't see a reason to go to Java 2.0. So really we are dealing with marketing spin. Which is fine. Marketing folks have to earn a living, too, so if they think that changing some numbers will create some interest before their big Java convention and get dopey people with blogs to write about it then more power to them.
So there. I have done my bit by spreading the new Java 5.0 meme using the mighty power of my blog. Mission accomplished, Sun marketeers!