The regime is NOT going to collapse. And that's not the goal of the vast majority of those marching Tehran's streets.
This is not about ending the Islamic Revolution, it's about getting back to it. For all his talk of returning it to its roots, Ahmadinejad's slow crawl from a de facto dictatorship to a de jure one is a shift away from the Revolution, which was, let's not forget, first and foremost about getting rid of the dictatorial and tyrannical Shah, not about Islam and that state.
Moussavi has made clear that the people are behind him not for his sake, but for the sake of the Republic that they love. Likewise, the emerging ritual of standing at windows, balconies, and rooftops at around 10pm and shouting "Allah-o-akbar" is a call of hope for the idealism of 1979 -- hardly a time an anti-Islamic Republic of Iran movement would look to. I think it's the most moving thing to come out of the whole ordeal so far:
Check out Trita Parsi's article on the same subject here on Time Magazine's website.