The WordPress developers have been working hard to keep to a more regular release schedule over the last year or so. According to the schedule, version 2.4 would have been coming out on or around 24 January. However, some of the planned new features, including a major overhaul of the admin interface were just not going to be ready by then. Rather than put out a release with very few real improvements, it’s been decided that version 2.4 will be an unversion. It has ceased to be before it even saw the light of day (or night, depending on your time zone). Work is now going ahead on version 2.5, planned for release in March.
There will more than likely be a few interim releases with security updates between now and the release of WordPress 2.5.