In 2006 the International Astronomical Union (IAU) adopted a definition of "planet" that from its outset, sparked controversy.
The IAU defined a planet to be:
A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
As planetary scientist David Grinspoon explains in the March 2009 issue of Sky & Telescope, many astronomers have voiced extreme displeasure with this definition, and are actively seeking to overturn it.
He goes into these issues in even greater depth with S&T's Robert Naeye in this 11-minute podcast.