Sky and Telescope magazine (www.skypub.com) had an interesting article a while ago where they tried to come up with a logical basis for defining a planet that would encompass all the current 9 planets while excluding others. They were unable to do so: criteria such as "massive enough to pull it's own mass into a spherical shape" or "within x distance from the Sun", or a specific chemical/geological composition all either excluded Pluto (and others), or else included other objects not currently considered a planet.
If you want to read more about this object, check here
I thought the definition of a planet was pretty simple. Orbits the sun, is big and round. Did I miss something?