Only four percent of the IAU voted on the controversial demotion of Pluto, and most are not planetary scientists. The vote was conducted in violation of the IAU's own bylaws on the last day of a two-week conference when most attendees already had left. No absentee voting was allowed. Supporters of the demotion resolution violated the IAU's own bylaws by putting this resolution on the General Assembly floor without first vetting it by the proper committee as IAU rules require. Also, many planetary scientists do not belong to the IAU and therefore had no say in this matter. When professional astronomers objecting to the demotion asked for a reopening of the planet debate at the 2009 IAU General Assembly, the IAU leadership adamantly refused. Why would they refuse to reopen a debate unless they were insecure about their stand? Meanwhile, this issue continues to be debated in other venues, such as the 2008 Great Planet Debate, held at the Johns Hopkins University Applied Physics Lab in August 2008 (which I personally attended), the American Geophysical Union, and the European Geophysical Union.
The IAU decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASAs New Horizons mission to Pluto. One reason the IAU definition makes no sense is it says dwarf planets are not planets at all! That is like saying a grizzly bear is not a bear, and it is inconsistent with the use of the term dwarf in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies. Also, the IAU definition classifies objects solely by where they are while ignoring what they are. If Earth were in Plutos orbit, according to the IAU definition, it would not be a planet either. A definition that takes the same object and makes it a planet in one location and not a planet in another is essentially useless.
Pluto is a planet because it is spherical, meaning it is large enough to be pulled into a round shape by its own gravity--a state known as hydrostatic equilibrium and characteristic of planets, not of shapeless asteroids held together by chemical bonds. These reasons are why many astronomers, lay people, and educators are either ignoring the demotion entirely or working to get it overturned. You can find out more by Googling "Laurel's Pluto Blog."
A decision should not be blindly accepted as some sort of gospel truth because a small number of people decreed it so. The IAU can decree the sky is green, but that doesn't make it any less blue.