I was wondering why they had to calculate its mass the same way they do with binary stars. I'm guessing it's because they don't yet know the orbital period P, or the semi-major axis, a. a is the distance between the two farthest most points on the orbital path, which is usually elliptical. They haven't said how far it is, and to the best of knowledge I think there is no way to calculate its distance from the sun until they can measure or estimate P using Keplers third law, and lets face it it could be hundreds of years long. I don't know if they can measure velocity from red shift using Hubble's Law accurately enough to calculate the period based on this, it probably never travels towards or away from the earth fast enough to give rise to a detectable red or blue shift.

Kepler's laws of planetary motion - Wikipedia, the free encyclopedia
Hubble's law - Wikipedia, the free encyclopedia

EDIT I score a C in my astronomy resits, d'oh. Here are the hard facts, I can't work out how they measured the distance yet though...

HubbleSite - NewsCenter - Astronomers Measure Mass of Largest Dwarf Planet (06/14/2007) - Fast Facts
Staurm Reviewed by Staurm on . Astronomers measure mass of largest dwarf planet HubbleSite - NewsCenter - Astronomers Measure Mass of Largest Dwarf Planet (06/14/2007) - The Full Story Rating: 5