it seems to me sadr is acting like lenin in october 1917. and i think this means it's possible that the result of the current phase of fighting may end with a political solution. let me explain.
as in russia in 1917, real political power is lying on the ground in iraq. the former ruler, who kept himself in power through fear and political police, is gone, but the institutions established in his absence (in the case of iraq, by occupiers) lack legitimacy.
sadr is heads a small, radical vanguardist faction that figured that it can come along and scoop up power with a focused use of force. once the fighting started, he calculated, not incorrectly, that other iraqis would be willing to overlook his ideology and join him. i suspect he figures fighters under control can force the u.s. to leave and then he can steer the country in his more radical direction.
in doing this he eclipsed al-sistani, who's a revolutionary in intent but more of a gradualist, committed, if not to constituionalism, at least to an initial attempt at non-violent action and `legitimacy' in the terms set by the u.s.
the problem is that he may have been eclipsed by sadr's decisive action, and may face the choice of being eclipsed or having to have his followers join the fighting. at the moment, he hasn't yet made the choice, and from i've read he hasn't yet been abandoned by his followers.
frankly, i don't think leaving iraq in the control of sadr is a desirable outcome. there's been a lot of rhetoric aimed in his direction, but it's pretty clear from the preponderence of what i've read he's a radical theocrat who will probably betray the non-islamist sections of the population who back him (the same way the secular left in iran was betrayed after the 1979 revolution there)
why i think a settlement is possible: i think that sadr is primarily interested in power -- stealing a play from the book of the most famous athiest group in world history is an indication of that. if so, it will be possible to cut a deal with him if he thinks he can't win by violent means and is being offered real power in exchange. are the bushies smart enough to offer him such a deal? unfortunately, i think their ideology prevents this.