An extreme forecast doesn't merely grab your attention; ironically, it may strike you as even more convincing than a moderate prediction. A classic psychological experiment at the University of Michigan showed that 54% of people preferred an extreme prediction about stock prices to a more-temperate one. They apparently believed that a forecaster must have high confidence and a solid rationale in order to justify making a dramatic prediction. So, while Dow 1000 may or may not be a good forecast, it isn't bad marketing for newsletters...
The more extreme predictions tend to be popular at turning points. Zweig points out that the book "Dow 36,000 hit the best seller list around the year 2000. The article also points out that for Dow 1,000 (a 90% drop) to happen earnings would have to collapse and price to earnings would have to drop to record lows.
One of the examples given is Pfizer. What would it take for it to trade 90% lower? Earnings would have to contract 70% and it would have to sell at a price/earnings of 2.3.
Zweig also points out that if Coca-Cola ever fell to a PE of 5, Warren Buffett would buy the whole company pretty quickly.