The signs are pointing to a critical convergence that, to be honest, is coming just in time. The world's will is aligning. Climate change, oil prices, pollution, growth, commuting - these and other factors are forming a message in society's mind that says "things are not good and they must be fixed." Businesses and governments, at the same time, are realizing that the changes needed to achieve sustainability are not going to be a drag on the economy but can actually be profitable while being good for society as a whole. Of course, the high and rising price of oil has something to do with this as well.
And technology is improving - finally - to the point that our remaining energy needs, after the 50% reduction in energy intensity possible via efficiency, can be cost-effectively replaced by renewables. Scientific and technical announcements just in the last two weeks - factor of 10 reductions in fuel cell and hydrogen splitting catalyst costs; new materials lighter, stronger, and cheaper than carbon fiber; and new ways to collect sunlight and convert it to electrical, thermal, and chemical energy - will, when available commercially, combine with all the other technologies that continue to stream out of labs and corporations, to drive the prices of sustainable energy down, down, and farther down.
So what we're seeing at this moment in history is a powerful combination - the will to change with the technical ability to make the change, and the understanding that the change is cost-effective and in many cases profitable.
And that's why I'm optimistic.