Iraqi bloggers central gives a good overview of all the theories.

Here is an edited down version. I recommend reading it all from the link above.

The Chaos Theory:

Perpetrator: Al-Qaeda In Iraq (aka Zarqawi/jihadis) or ex-Saddam Regime criminals (aka Saddam's Orphans) or both (aka The Return Party).

Purpose: To start a civil war for the purpose of general mayhem in the midst of which they can jettison the Iraqi democracy as unworkable and step in as Iraq's nationlist, unity, and security dictatorship and protector of morality.

Who believes this story: Hammorabi does:

"The barbaric and savage attack on the Shire of Imam Al-Hassan Al-Askari in Samara is a continuation of the barbarism of the Saudi Wahabi terrorism, which started such destruction against the entire ancient heritage."

Hassan of An Average Iraqi believes it (I think so, the alternative would only make me sad):

"I believe that the reason for such an attack, is an attempt to stir up civil war, between the Shias and Sunnis. In addition, of course, Al-Sadr is not helping at all...Does he not notice that he is falling into the plain trap laid in front of him?"

Also, the US military sees the Return Party's hand in the bombing:

[Col. Jeffrey Snow of the Multinational Division Baghdad said] "You know, the terrorists would like to see this break out in civil war."

...and in the spreading of false stories of mosques being destroyed.

US coalition spokesman, Major-General Rick Lynch... also suggested that the figures on attacks were exaggerated: "We believe there are people out there generating false reports... making things seem worse than they are. There have been pockets of violence, but we don't see that as a precursor to civil war." [He added] that US-led coalition forces have increased their presence alongside Iraqi security forces to defuse the situation.

In every way this attack benefits the same people who have been engineering chaos in Iraq since April of 2003....

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The Reichstag Theory:

Perpetrator: The Mehdi Army (aka Sadr's Thugs)

Purpose: To have a cause-celeb to rally their faithful, sweep them into power, and purge the Sunni Arabs and other infidels from Iraq. In 1933 Germany, just before an election in which the Nazis were running on an anti-Communist platform, Hitler sent his thugs to set fire to the German government building and symbol of its democracy, The Riechstag. The Nazis blamed the Communists and used the event to sweep Hitler to power. There is a lot to recommend this theory: All the stories of black pajamas attacking Sunnis all over Baghdad. It has a proven history of success.

Who believes this story? Errr...hard to say. Everybody and nobody. It's doesn't seem to be in any Iraqi's interests to buy this story. Even those who see Sadr as culpable prefer to see him as an arm of the Jaffari's Internal Ministry or Iran. But there are some exceptions: IraqPundit seems to see Sadr behind this....

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The Government Plot Theory

Perpetrator: The "Iran-friendly" UIA/SCIRI Iraqi Government

Purpose: Umm...I don't know exactly. Maybe they want Iraq partitioned? Maybe they want a Sunni purge? Revenge over Saddam? There are as many potential motivations as there are Shi'a.

Who believes this theory? Maybe Zeyad (who, along with ITM, is doing a yeomans job of documenting the mayhem)

In reply to my question about whether the al-Qaeda is generating false reports of attacks on mosques to fan the flames of a civil war, he said:

CMAR, you don't seem to get it. This is not a Qaeda vs. Iraqis conflict anymore, it's a Shi'ite dominated government vs. Sunni community conflict. These are not false reports of attacks I am posting, these are very well documented attacks with pictures and eyewitnesses from different areas of Baghdad and Iraq. If you understood Arabic, I would have directed you to Iraqi local tv channels which continue to receive mayday phone calls from citizens in Baghdad, except the governmental channels of course. There is fighting outside in my area. Now they are using mortar shells. The US military is nowhere to be seen. And perhaps that's a good thing. They can't afford to be forced to side with one group over the other. Just try to get this whole Al-Qaeda/Saddam orphans hype out of your head for a moment.
That time is over.

This statement doesn't necessarily mean he buys the theory though. He might only think the government is exploiting it. But many Iraqis do....

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The Tehran Theory

Perpetrator: Iran

Purpose: See Theory 3: The Government Plot.

Who believes this theory? Reportedly many Iraqi Sunnis do. Honestly, it is hard for me to distinguish this theory from the "Government Plot" Theory or the Riechstag Theory since it always seems to come back to SCIRI or Sadr or both as co-conspirators, but perhaps the line is starker for some Iraqis. Maybe not, though (from the linked article):

The Iraqi National Accord party, headed by former Iraqi Prime Minister Ayad Allawi, a secular Shiite, accused the pro-Iran Shiite coalition United Iraqi Alliance and the followers of the firebrand Shiite cleric Moqtada al-Sadr of carrying out revenge attacks on Sunni mosques during which over 100 people were left dead.The Iraqi Islamic Party had a similar position. The party’s secretary general Tareq al-Hashemi told the al-Arabia satellite channel on Thursday, “The Shiite coalition and the followers of Sadr as well as their foreign supporters are behind this aggression”.

The IIP has been a big booster for the insurgency up until now. So it should surprise no one that they see everyone, and I mean everyone, behind this plot except the insurgency...even to the extent of imagining that Iran worked with the US military to pull this off. Which leads us to The Great Satan Theory.

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The Great Satan Theory

Perpetrator: The US Military/CIA/Mosaad

Purpose: Divide and Control. Get the Shi'a and Sunnis fighting each other as a distraction and then the US can sneak the oil out of the country.

The great thing about this theory is that it requires no deep thinking. It is founded soley on another brainless premise a) that the US is behind every evil act that occurs in the world and b) that everything but everthing is in its interest except for the decent people of the world to rise up and throw off the chains of American Imperialism. Since the US has spent a lot of blood and money forming a democratic government (that supposedly is peopled by its puppets), one would normally presume that sectarian war would be the worst thing the US could imagine. But nooooooo, not if you are the Great Satan (see my published conversation between the Great Satan and an "in-the-know" Arab)

Who believes this theory? Wackos. Fools. The bloggers' equivalent of the Heavens Gate cultists. And most of the Arabs in Egypt, the West Bank, and Jordan.

Even Raed Jarrar is hawking this theory. Remember that, from the very beginning, Raed has been predicting an Iraqi-on-Iraqi civil war that would force the US from the region (I'd love to link to the first instance of it, but since Raed's move to one of the most expensive cities in the US, the post has faced a Stalin-like purge). But now? Oh no! This isn't the one he was talking about. This civil war is the US's doing. Eagle at Truth has posted on an interview Raed gave at Democracy Now! Eagle thinks this theory is so ludicrous even DN! doesn't want to be associated with it. I'm sorry, but I'm not believing that there is a Great Satan Theory that DN! won't buy.

Ladybird and Truth-About-Iraqis are flogging this theory too. You know, maybe you can't learn much about the quality of a book by who is quoted on the blurbs. But I believe the opposite is true for conspiracy theories.

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"It Doesn't Matter Who":
Iraq Is Lost Camp vs Iraq Will Succeed Camp

The "Iraq Is Lost" conclusion is founded on the premise that "a civil war among the Iraqis is inevitable". This has been a popular theory among the White Flag Democrats and the Lindberg Republicans. It has also been a popular theory among anyone in the world who has wanted Iraq to fail. It's convenient. If Iraq's failure is inevitable; if the Iraqis have "a right" to a civil war; then it is not "defeatism" to say that no one can save Iraq from disaster and the best thing is for the US to get out of the way, stop prolonging the inevitable, and let the bodies in Iraq pile up as high as they can so they can be laid at the feet of George W. Bush. Hahahahaha!......