Roy Spencer has a nice essay on sustainability in TCS daily. The only sustainable thing is change, he says. He also argues that if the consumption of oil or production of carbon dioxide were unsustainable, a slower rate of the same processes would be unsustainable, too.
Sustainability becomes irrelevant because of technological advances in almost all cases. Spencer chooses Michael Crichton's favorite example - the unsustainable amount of horseshit in New York City 100 years ago when there were 175,000 horses in the city. Its growth looked like a looming disaster but it was stopped because of cars that suddenly appeared.
Also, he notices that the employees of a British Centre for Ecology and Hydrology - that had to be abolished - were informed that the center was unsustainable which is a very entertaining explanation for these people who fought for sustainability in their concerned scientific work. Also, Spencer gives economical explanations to various social phenomena. For example, the amount of possible catastrophic links between our acts and natural events as well as the number of types of our activities that will be claimed to be "unsustainable" in the scientific literature is proportional to the amount of money we pay to this sector of science.
It looks like we can run out of oil soon because the companies have no interest to look for more oil than what is needed right now - it is expensive to look for oil. That makes it almost certain that we will find much more oil than we know today.