Demonstrating that a planet is Earth-like would require far more sensitive instruments than planet finders use currently because you would have to constrain the composition as well as the mass and size of a body which is currently below the threshold of detection. Of course just a minimum mass of ~5 and an irradiation 20% greater than Venus' for Gliese 581c was enough to provoke media reports that Earth's "twin" had been found, and spawn hundreds of posts here about its likely oceans, biosphere, plans for missions and colonies etc, so I expect numerous more such outbreaks of mass, uh, optimism before the real thing is found.