Al Giordano's analysis of goings-on in Iran is worth reading. For a different view, there's George Friedman of STRATFOR:
"Successful revolutions have three phases. First, a strategically located single or limited segment of society begins vocally to express resentment, asserting itself in the streets of a major city, usually the capital. This segment is joined by other segments in the city and by segments elsewhere as the demonstration spreads to other cities and becomes more assertive, disruptive and potentially violent. As resistance to the regime spreads, the regime deploys its military and security forces. These forces, drawn from resisting social segments and isolated from the rest of society, turn on the regime, and stop following the regime's orders. This is what happened to the Shah of Iran in 1979; it is also what happened in Russia in 1917 or in Romania in 1989.
Revolutions fail when no one joins the initial segment, meaning the initial demonstrators are the ones who find themselves socially isolated. When the demonstrations do not spread to other cities, the demonstrations either peter out or the regime brings in the security and military forces - who remain loyal to the regime and frequently personally hostile to the demonstrators - and use force to suppress the rising to the extent necessary. This is what happened in Tiananmen Square in China...."
Friedman makes some good points, while others strike me as less well grounded. Read him and decide what you think.
Friedman talks about revolutions succeeding or failing. He overlooks a third possible outcome, one that has repeated itself many times throughout history: namely, the revolution being effectively hijacked by some faction out of step with the original revolutionary leaders, but more ruthless and committed to winning by any means necessary - so that when the government is overthrown, what takes its place is not what the original revolutionaries had in mind, and may in fact turn out to be worse than what it has replaced.