To be named a planet, there are 3 conditions to be associated with it..
It has to be in orbit and around the sun
Has to be big enough to be a sphere with it's own gravitational force
Lastly, it has to have gravitational dominance in it's own 'orbit zone'..
I think that was the one that Pluto failed on. The last one.
In 2015 we'll know more about Pluto since the New Horizon spacecraft would have reached it and will have investigated it more.