I think the best definition I've seen for planets is the one provided by the folks at CalTech who took part in discovering Sedna: "We define a planet to be any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit. For example, many asteroids cross the orbit of the earth. Yet the earth is more massive than all of those put together. Thus, the earth is a planet. Ceres, the largest asteroid, is not greater in mass than the sum of the masses of the remaining asteroids. Hence, not a planet."
Sedna almost certainly doesn't meet this criterion.
And I still think Cruithne is a lot cooler. (See also: Paul Wiegert's page about Cruithne. Scroll to the bottom and check out the movie of its orbit.)