The International Astronomical Union is almost ready to ratify rules on what constitutes a planet. Over the years, as more celestial bodies are discovered in our solar system, the debate about Pluto's status as a planet has been gaining momentum. As always the Bad Astronomer has the inside scoop on how the process is going. His conclusion: we're probably going to end up with 12 planets, and Pluto is going to be classified as a "pluton":
We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”.
I like this approach - if it doesn't fit neatly into our existing "planet" category then we create a new category. Even though their definition is inexact I think it is a step in the right direction.
If this interests you at all I highly suggest that you read the Bad Astronomy article - it's a fascinating discussion that this little post doesn't do justice.