At the 2006 IAU meeting held in Prague this past summer, the scientists voted that every planet must also have "cleared the neighborhood around its orbit." Since only spheres with a large mass can achieve such orbital dominance, Pluto was no longer a planet. The scientific bureaucracy had spoken; our solar system had shrunk.
Too bad for Pluto. But in the end I think this will turn out to be one of those nitpicky things only people like me know and bore everyone with.
"Did you know that technically a tomato is a fruit?"
"Is that right?"
"Yes, a tomato is technically a fruit but it was ruled a vegetable for tariff purposes. Hey, wake up!"
"Did you know Pluto isn't really a planet because it doesn't clear the neighborhood around its orbit?"
Pluto will stay a planet in everybody's mind because it's always been a planet. The IAU should have just grandfathered in Pluto, saved everybody a bunch of trouble, and made themselves heroes to every kid with even a passing interest in space (which should be a lot more of them).