Trying to get away from the media hype surrounding the announcement of the discovery of the new planet orbiting Gliese 581, I learned that there are some important details that are not mentioned in the popular press.
First of all, there's not much certainty about most of the features that would make this planet habitable (at least, habitable to us). What scientist know for sure, at the moment, is the planet's mass, orbital period and distance from its star. The radius of the planet is pretty much a guess based on assumptions about its density, and the idea that it is a rocky/watery planet comes from the same assumptions. The gravity at the surface depends on the same data, of course.
Also, the surface temperature seems to be a very wild guess. After all, the temperature of a planet depends not only on the amount of energy it receives, but also (and mainly) on how much energy it retains. So, the planet's albedo and its atmosphere density and composition play very important roles, and we know nothing about any of these. The temperature range commonly given (0 to 40°C) assumes an albedo similar to Venus's, and I think we all agree that Venus is not quite habitable. If you assume an atmosphere and albedo similar to Earth's, the average temperature is some -17°C (which is still not terrible, and might allow for liquid water in some locations, but is not very inviting; think Hoth).
One extra point is that the planet is so close to the star that it is almost certainly tidally-locked to it; that is, the same side of the planet faces the star at all times (just like the Moon does in relation to the Earth). What impact this would have on the climate of a planet with a dense atmosphere (if this turns out to be the case) is anyone's guess, but the current consensus seems to be that the effect wouldn't be too bad (winds would probably even out the temperatures). It would be an interesting planet to live in, though, with a sun hovering in the same piece of sky all the time (I guess species evolving in this kind of planet would not have a circadian cycle; I wonder what this would do to the inner workings of brains).
None of this makes the discovery any less interesting and important, but it's always a good idea not to get too excited with the information you see in the media. We may get more information (say, about the size and composition of the planet) when it transits in front of the star as seen from here; the next such event is expected to happen next week. As more people look at this system and more information is gathered, we'll certainly get a clearer picture of what life would be like in Gliese 581 C.