So that’s that then; Pluto is no longer a planet in the old sense of the word. Now for something to be a (classical) planet it has to satisfy three conditions:

It orbits around the Sun
Has sufficient mass that its own gravity overcomes rigid body forces so that it assumes a hydrostatic equilibrium (roughly spherical) shape
Has cleared the neighbourhood around its orbit

The new category of Dwarf Planet has the same first two properties but has not cleared the neighbourhood around its orbit. This means that there are at present four dwarf planets in our Solar System by my reckoning:

Pluto
Charon (Pluto’s moon)
Ceres (formerly just an asteroid)
2003 UB313 (as yet unnamed)

So although I’m disappointed by Pluto’s relegation, I am at least pleased that we got a new branch on the celestial family tree. I think that reclassifying and reordering the Solar system to 12 bodies would have been tricky to justify and a little complicated. But the simple demotion of Pluto makes some sense. By adding the new Dwarf category, the IAU has made things more interesting in the long run. Although I wouldn’t want to be the guy that has to edit al those children’s textbooks.