Sun has decided to rename J2SE 1.5 to J2SE 5.0, as this is a very significant relase with important changes to the Java platform. According to this The Server Side posting, Sun also considered getting rid of the “2” and going with J5SE instead, but this was not a good choice because of the J2EE brand.
I think Sun made a bad decision when they released J2SE 1.2, the first Java 2 release. They should have called it Java 2.0 back then, without the confusing dual versioning.