If you (scientist) discover it I think you want to call it a planet. If you (scientist) didn't discover it you want to call it an asteroid.
From what I have read Pluto and all these more distant objects discovered thus far are at best merely large asteroids. I am not 100% sure on this but I believe Pluto is not a true spheroid which I believe really defines a planet versuses an asteroid.
When you look at "moons" however you get a whole new monkeywrench. The moons in our solar system are either "planets" or asteroids that were captured by larger planets which now orbit around a common center of gravity. Are own "moon" is the largest moon relative to the planet it orbits in our solar system. Many scientist think it is more accurate to call the Earth/Moon a binary planet system. If our "moon" was in orbit purely around the sun and not with the Earth it would no doubt be considered a planet.
The "moons" of Mars are obviously asteroids.
Many of the "moons" of the larger planets should be considered captured planets. The "moon" Europa is probably the best place to look for life in our solar system outside Earth.
In the book "Pale Blue Dot" by Carl Sagen (Which I recommend if you like astrophysics), Carl Sagen overcame the semantic issues of planets versus moons by calling them "worlds".
I read that there is still some thought that a large planet that orbits our sun may still be out there at an extreme distance and lay undiscovered. Although not found it would explain some issues with gravitional perturbations that some scientist believe still exist when calculating current planetary and solar orbits.
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