In addition to "downgrading" Pluto's status, the International Astronomical Union also defined two celestial objects - the asteroid Ceres, Charon (Pluto's moon) and 2003 UB313, nicknamed "Xena" by its discovered Michael Brown of Caltech - as dwarf planets.
Michael Brown (the astronomer, not the FEMA guy) wrote (before the astronomer's vote) that calling Pluto a planet would cause many other celestial objects to fall into that category:
While most people would answer that there are 9 or perhaps 10 planets, a proposal by the International Astronomical Union that will be voted on soon would significantly increase the number of objects that astronomers call planets. The proposal is to call any object that is large enough to make gravity cause it to become round a planet.
How many planets would this make? The nine planets that everyone knows are all round, so they are clearly planets. Ceres, the largest asteroid, is also round and would become a planet (the fifth). The big question, then, is how many new planets are there in the Kuiper belt, a region of rocky/icy bodies beyond Neptune, and the home of Pluto and 2003 UB313 ("the 10th planet").
For now, the number of known objects in the solar system which are likely to be round is 53, with the number jumping to 80 when the objects from our survey are announced, and to more than 200 when the Kuiper belt is fully surveyed.
And the solar system would look like this:
So, despite public outcry, astronomers voted to kick Pluto out instead of letting in hundreds of space rocks into the Planet Club.
Link: Michael Brown's Website
This cartoon, drawn by Richard Crowson of the Wichita Eagle, Kansas - source: the always awesome Daryl Cagle's Professional Cartoonist Index.