So now we have a conundrum. Sedna may be bigger than Pluto and may have a moon. So there are only two options for the planetary catalogers.
1) Sedna is not accepted as a planet. But then Pluto might have to lose its designation as well, knocking us back to 8 planets. That's especially probable if a bigger planetoid is found, which is looking more likely every year.
2) Sedna is accepted as a planet, moving us up to 10. However there might be many, many other planetoids as big as or even bigger than Sedna that we haven't seen orbiting the Sun within the Oort. Do we then add the next 20 or so of them as they're found?
It's like the situation with the gas giant moons: Voyager discovered new ones, they were added, but it was later found that there were 30+ rocks of the same size. Where do we draw the line? Whats a planet and what's not?
