From a commenter on Juan Cole's blog posting last December:

It is impossible to scientifically measure what portion has the surge played in improving security. So many things changed in parallel with it: the Awakening groups who are the only people who succeeded in kicking al-Qaeda out; the Mahdi Army ceasefire; the success of ethnic cleansing; the restoration of Iraqi national pride (against the wishes of the Americans) etc.

Regarding the Mahdi Army, they apparently were in ceasefire in December, but since March things have gotten worse again. Lately the Iraqi government has been waging a campaign to disarm the Mahdi Army which began in March with the Battle of Basra.

Baghdad appears to have gone from 90% sunni to 40% Shia since 2003, and one of the ways that the US occupation forces are trying to control the sectarian violence is to build a wall separating the Sunni from the Shia.

On 22 April, Prime Minister Nouri al-Maliki called for the building work to cease. Subsequently, on 23 April, an estimated 7,000 Iraqis engaged in a peaceful demonstration against the wall, several carrying banners reading (in English) "No to the sectarian barrier."

Following the demonstration, the U.S. military issued a statement that "the construction of the wall is under review" and that they would "coordinate with the Iraq government to establish effective appropriate security measures." However, at a news conference later on the same day, spokesmen for the U.S. and Iraqi military stated that they had no plans to stop building temporary separation barriers, with Brigadier General Qassim Atta describing the media reports that the Iraqi Prime Minister was protesting about as "groundless."

At the news conference, Brigadier General Atta said: "The prime minister is in agreement with the work of the security forces and the issue of security barriers. We will continue to set up these barriers in Adhamiya and other areas." According to Atta, the barriers -- which were to consist, he said, of sand barriers, trenches, barbed wire and concrete barriers constructed from moveable sections each weighing 7.1 tons (6.3 tonnes) -- would be only a temporary measure, to secure specific areas of Baghdad, and would be moved once each area was considered secure.