Creek Running North has said everything I wanted to say about the Hawking statement--and has said it much better. One excerpt:
Hawking’s future of humanity resembles nothing so much as those aliens from Independence Day, the ones that travel from planet to planet laying waste the resources and moving on, pausing only to strangle Brent Spiner. OK, so every disaster has an upside. My point is that our planet deserves better than to be a mere discarded larval skin for a species that goes on to eat the galaxy, implacable and soulless and vulnerable to computer viruses written in haste by a drunkard, on a Macintosh.
Though I agree with the sentiment, I actually think Chris is being a teensy bit harsh here. Hawkings wasn't asking us to neglect the Earth. Even so, I do find the idea that it would be easier for us to travel to another star, find a habitable world, and make it habitable for humans, than it would simply to develop a sustainable pattern of life right here a bit farfetched. When we consider the time and investment involved, a sustainable future on Earth is probably the necessary basis for such a journey.