I'm a little torn on this issue. Pluto has always been a planet to me and I wouldn't want to see it go. Regardless of all the new minor planetary bodies out there that approach its size, I would prefer to see it stay one of the major planets.
I guess that's why this poll included the option of voting other major bodies into the fold. Some very large bodies like Sedna, Quaoar, and 2004DW approach Pluto's size. This makes the border between planet and non-planet seem arbitrary. But, in a sense it is. Like the border between planet an brown dwarf, there is plenty of gray area. The distinction between brown dwarf and star is obvious; is nuclear fusion going on? If yes, then it's a star, if no then it is a planet. Brown dwarfs fill this upper niche of planetary objects. But the distinction between brown dwarf and ordinary gas giant planet is less clear.
The other end isn't so clear. The we have Pluto and the other wannabes in close proximity of size shape and other properties. This makes the decision seem arbitrary and non-scientific. Much like the border between Europe and Aisa. It's just a line drawn in the sand. So clearly, without well defined criteria, the distinction between the two groups would be mere convention.
This convention began, I suspect, when Gallileo first turned a telescope to the skies. First moons were discovered orbiting Jupiter, then later other moving bodies were found in the skies. Originally, there were 7 "planets:" the moon, the sun, Mercury, Venus, Mars, Jupiter, and Saturn. Basically anything that moved in the sky was a planet. Hence the meaning of the name: "wanderer." Then other bodies were found. Comets were discovered to be icy bodies that grew tails when close to the sun. Asterioids were rocky/metalic bodies with any volatiles boiled away long ago. An moons were easily catagorized as large rocky bodies orbiting a planet. For a long time, comets and asteroids were designated "minor planets" or sometimes "planetoids." However, that terminology faded away.
When the new planets Uranus and Neptune were discovered, they fit the planetary mold nicely. Nice, nearly circular orbits that fit snugly within the plane of the other planets. Easily visible disks as seen through the larger telescopes. And even moons circling each planet. They were found by tracing gravitational anomolies to their source. Then observing photographic plates for changes. When one of the "star" moved, it was determined to be the planet.
Pluto was a little different. It was discovered by tracing gravitational anomolies like Uranus and Neptune. Only, it turned out not to have the mass required to cause the effects they were looking for. It also had a much more elliptical orbit than the other planets and its orbit was tilted by a noticable angle. As a matter of fact, it even passes within Neptune's orbit for part of its orbit. And it is so small that for a very long time, we could resolve no more than a bright point.
Regardless, according to the Planet Society's poll, a slim majority would like Pluto to stay a planet. Though, there is a significant minority who thinks it should be downgraded.