Sun announced that the upcoming J2SE release (1.5, aka 'Tiger'), will be officially called 'Java 5', skipping multiple version numbers.
It doesn't give any reasons for this, though David Flanagan, who broke this story, thinks its marketing driven; perhaps 'Java 5' will draw more attention than just a increment from 1.4 to 1.5.
Java 5 does have lot of new things though- Generics (similar to C++ templates), 'Enhanced' for loop, Autoboxing/unboxing etc.