Well, they finally found it,
Planet X, a.k.a. Sedna. Of course, if it is decided that it is actually a planet, the name will have to be changed to a Roman god. Sedna is the Inuit goddess of the ocean, and the Roman god of the ocean - Neptune - is already taken.
Interestingly, the Discovery Channel website reports (http://dsc.discovery.com/news/briefs/20040315/planet.html) that the body appears to have a companion (read:
moon), which is the second reddest object in the solar system, after Mars. Wow, these NASA fellas can even measure redness accurately! Since the rules of moon-naming are less strict than those for planet-naming (or rather there's a justifiably greater set of allowed names), perhaps this moon will end up being called
Of course, the big question is
is it a planet? Well, if size is your criterion, most probably yes, given that Pluto is officially a planet and Sedna is (expected to be) not much smaller, and may even be bigger than Pluto. But size isn't the best criterion - Pluto is actually smaller than the Moon, and the Moon's not a planet (although astronomers sometimes describe Earth-Moon as a double-planet because of their relative sizes).
There's some debate over whether Pluto should qualify as a planet - it's usually described as a Kuiper Belt Object (KBO). (The Kuiper Belt is a field of small, rocky bodies orbitting beyond Neptune - essentially a second Asteroid Belt.) Pluto, Sedna, Quaoar et al. are rightly not classified as asteroids - they're too big - but they're basically the same thing... just bigger. The truth is there's really little intrinsic difference between the smallest of space rocks (meteorites and the like) and much larger objects like Mercury and the Moon.
Sedna orbits the Sun at three times the distance of Pluto (taking a whopping 10,500 years to do so). If Sedna is actually a KBO, that means the Kuiper Belt must be absolutely colossal. If it's a KBO, it's as much a planet as Pluto. If it's not part of a belt, then I think it must qualify as a planet.
NASA will give a press conference at 18:00 UTC.