As discussed in the developer mailing list, the next release of WordPress will be in March instead of January because of the holidays and the amount of changes that will take place in the codebase as well as the admin section. The decision was made to consider 2.4 a skipped December release and move straight on to 2.5. The reasons for the change in the schedule include some good things cooking in the oven and the developers do not want this to become a rushed release.
So to sum things up. There will be no 2.4. Instead, we will see 2.5 which is scheduled for release in March. Various official docs and roadmaps will be updated in due course.