From the blog "Can't Keep Quiet" by Zaneirani:
The arrests made by the coup d'état government yesterday has no significance other than a show of force. Rafsanjani is the only person who, as the head of the assembly of experts, has the authority to remove Khamenei from power. He has the will but the question is whether he has enough number of votes (i.e. enough support among clerics). Many believe that he is in Qom to make such assessment. Additionally, such high risk move has a chance of success only against the backdrop of mass dissatisfaction with the election results and at least some support from the armed and security forces.
A velvet change is underway. Whether it turns out to be a velvet revolution or velvet coup d'état remains to be seen.
A similar view of the critical role of Rafsanjani is expressed in this Letter from Iran posted at Salon:
It's becoming increasingly clear that this was a palace coup, a palace coup in the style of Peru's Fujimori. The Guardian Council has to accept the election results. All eyes are now on Hashemi Rafsanjani, who has apparently just resigned as chairman of the Expediency Council. He was the sole member of the original "yaran" of Khomeini, or Khomeini's original team, with power and influence. Hossein Mousavi is under house arrest.