An Earth-Like World?

Gliese 581g has a mass about three times that of the earth and it orbits a red dwarf star not too different from our sun. According to its discoverers, the planet is the right distance from its star for liquid water. And it's only 20 light years away. And I think this is pretty cool, but I don't share the discoverers' certainty that the planet must have life:

"Given the ubiquity of water, it seems probable that this thing actually has liquid water. On the surface of the Earth, everywhere you have liquid water you have life," Vogt added.

The question wouldn't be to defend that there is life at Gliese 581g, says Butler. "The question," he said, "would be to demonstrate that there isn't."

Since we have only studied one planet with liquid water, this one, it seems quite a stretch to me to assume that life will evolve on every wet world. We have no data on the question.