Sun will be dropping the project names they use for upcoming JDK releases to get ready for the open sourcing of parts of the Java platform. I guess this is to reduce confusion about what project name is what release.
So from this point onwards, the release formally known as Mustang will be called just JDK 6, and Dolphin will be just JDK 7. Both projects are already on java.net – JDK 6 is here, and JDK 7 is here.
For a small piece of Java history, check out this site which lists all the project code names for all the previous Java releases.