That famous quote is often misattributed to Yogi Berra, but in fact comes from the French poet Paul Valery, who was describing the sentiment among European intellectuals, in the 1920s, who looked out over the physically and morally devastated landscape of post-WWI Europe and encountered nothing but pessimism, of which Oswald Spengler's morbid ruminations were to become the most renowned.
Joel Garreau has an interesting piece in today's Washington Post about the decline of Americans' willingness to think hard about the future. He proposes that the reason for this decline is that the pace of technological change is so dizzying that people feel overwhelmed. He quotes Danny Hillis as saying, "We are future overwhelmed. I don't think people try to imagine the year 2050 the way we imagined 2001 in 1960. Because they can't imagine it. Because technology is happening so fast, we can't extrapolate."
The article is worth reading, but I wonder about the thesis. It seems to me, rather, that Americans' optimism/pessimism about the future (and about their ability to prognosticate the future) is a tightly correlated dependent variable of how Americans feel about the current state of domestic affairs in the United States. Thus, for most of the post-Depression era, Americans have been optimistic about both the content of the future and about our ability to predict things about it. The two exceptional periods have been "the long 1970s" (1968-1983) and the current post-9/11 era. During both of these latter periods, the dire state of affairs at home produced a (melodramatically, dare I say) generalized pessimism about the future as a whole.
What remains constant in both the optimistic and pessimistic phases, however, is Americans' willingness to extrapolate wildly and globally from our own current mood.