Wishful thinking: The Soviet Union and its ruling ideology, Communism, which spread over nearly half the world, collapsed with minimal fighting and deaths in 1989 - 1991, leaving a number of democratic governments in charge. Therefore, with some good luck, later revolutions (condescendingly modified with a word of others' choice) can follow the same pattern.
It's not impossible, of course, but one needs to go beyond the wishful thinking that Paul Wolfowitz et al. applied to the invasion of Iraq and look at some of the history of those regions.
The wishful thinking analogy, of course, is being applied to the Maghreb and wherever street demonstrations are going to shake the Arab world. Joshua Tucker provides a comment along those lines, and Kevin Drum questions some of what Tucker is saying. As I compose a response to Tucker in my head, it's becoming obvious that I might write all day on the subject. But let me keep it brief.
Tucker's point 1 is simply wrong. A great many people saw that Communism was unsustainable economically and even militarily. The Afghan war and environmental horrors in the satellite countries and the Soviet republics were causing public resistance for some time. It's true that nobody could predict that the Soviet Union would be officially dissolved on the precise date December 25, 1991, but many people in the US State Department and Europe saw it coming.
Tucker's point 2 is probably correct, but its relevance to the downfall of Communism is questionable. It was not demonstrations in the streets that brought Communism down all by themselves. The satellite countries and many republics had histories of independent rule before the Soviet takeover, and political parties were organizing and acting long before 1989. The legislatures were slowly, from perhaps 1980 on, transforming themselves from supreme soviets to parliaments, and many of their members were becoming more nationalist than Soviet. It's doubtful that such political underpinnings exist in the Arab countries.
Drum makes a good case for the importance of Mikhail Gorbachev's role in the downfall of Communism. Such actions, avoiding bloodshed, may or may not be taken by Arab rulers. Ben Ali did flee Tunisia without incurring large amounts of violence.
Finally, it's not clear what the outcome of the Tunisian revolution, the only one to oust a ruler so far, will be. Revolutions can go bad without the preparations that were taken in 1980s Eastern Europe. Lenin and the Bolsheviks took over in Russia in 1917, and the French Revolution got ugly. Ceaucescu's rule in Romania was ugly enough that things got bloody there, although Romania came through as a democracy. And not every Soviet republic managed to become democratic; look at Belarus.
Update: Maybe it's more like the Iranian revolution of 1979.
Second Update: Jim Lehrer can't resist. But Vice President Joe Biden has a good answer.