Ask yourself who benefits from killing the most influential and powerful Shi'a cleric in Iraq?
1. Moqtada Sadr. He wants to be the symbolic face of Iraqi's Shi'a, to inherit the mantle of his also-assasinated father. However, he has no political base capable of exerting real control countrywide.
2. Al-Qaeda. Shi'a are the essence of evil - accorded more hatred than Jews in the Salafist-Wahabi mindset. The threat of a Shi'a-dominated Iraq is an anathema to them, marking the end of Sunni dominance.
3. The pro-Ba'ath resistance. They know that Saddam is gone but they remember their power as the ruling monoparty of an autocratic state. They also have great reason to fear political ascendance of the long-oppressed Shi'a majority.
4. The Interim Governing Council. The IGC is full of former exiles and power brokers, who have been appointed to the supremem position of sovereign power within Iraq by the US forces. They also expect that, given the Bush Administration's election-driven timetable of sovereignity transfer by the end of the summer, they will be in a good position to retain most of the political power they now wield. They have been making their plans at feverish rates accordingly to cement their advantage.
5. The Bush Administration. Sistani's call for elections is a massive obstacle, which is ot easily circumvented because it appropriates the very language of democracy-building that formed the basis for war's rationale, especially in light of the fact that the WMD premise has been definitively shown to have been a sham.
With enemies like these, you might want to wonder who Sistani's friends are. That answer is simple: the theocrats of Iran. It would be irony indeed if Sistani's vision of direct democracy for Iraq could only occur via alliance with the theocracy of Iran. It gets more irony-laden when you consider the history of the US and Iran, and how the theocracy came to power there in the first place.
UPDATE: Don't miss the report from Juan Cole, on how Iran's reformers look to Sistani for support, and more background on the role that each of the power players have in the jockeying for Iraq's future. Sistani is the key to a truly free and democratic Iraq. He also personifies the difference between an acceptable outcome for neo-conservatives vs neo-wilsonians.
UPDATE 2: seems that there is doubt as to whether the assassination attempt actually occurred. Juan Cole mentions via email this report from a correspondent in Iraq:
"I just returned from 3 days in Najaf, Kufa, and Karbala. I did not hear anything about Sistani being attacked, and this certainly would have been news. I actually went by Sistani's house Thursday evening (around 6:30pm) and things were very calm."