The Hubble Space Telescope team announced the discovery of a fourth moon in orbit around Pluto. For those keeping track at home, Pluto’s previously-known satellites are Charon (discovered in 1978), Nix, and Hydra (discovered in 2005). Charon is quite large compared to Pluto, so much so that they mutually orbit a point somewhere in between the two objects — they are arguably a binary system. Nix, Hydra, and the new moon are tiny by comparison, which explains how hard it was to spot them.
Does this change anything about Pluto’s status? Probably not. An object doesn’t have to have moons to be a planet (or Mercury and Venus would fail that criterion), nor again is the number of moons relevant (since Pluto passes both Earth and Mars in number of satellites). Some asteroids also have moons, and Pluto’s fellow dwarf planets Eris and Haumea have known satellites. Pluto, Eris, and friends live in a region of the Solar System known as the Kuiper Belt; a standard theory is that a collision during the early history of the Solar System broke pieces off Pluto to make its satellites. Since there are so many little chunks of icy rock in the Kuiper Belt, that idea seems very plausible (at least to someone like me with no planetary science credentials!).
In any case, we’ll learn a lot more about Pluto and its moons — however many there might be — when the New Horizons probe arrives in 2015.