Add to that a very professionally done IED attack in eastern Baghdad near Sadr City with two seperate bombs going off right next to each other that killed four U.S. soldiers and wounded two others.
A demolition team that searched the site after the attack found an explosively formed projectile, a type of high-tech bomb that the U.S. military believes comes from Iran. (In other words, a shaped charge.)
Although I have been skeptical about whether these shaped charges and the knowledge of how to make them come from the Iranian government, per se, it does seem quite likely to me that the people who are perhaps more likely to have this skill would be Shi'a, rather than Sunni, and that they are getting assistance from someone(s) who quite possibly are Iranian.
So, let's theorize that there is a Shi'a faction out there with close ties to people in Iran, who are willing to target Shi'as who are collaborating with the enemy, and who are going to use somewhat more advanced weapons and tactics to make "the surge" a costly one. This group could be disgruntled ex-Sadrists, for example. If the goal of such a group is largely to pick off collaborators who have helped facilitate the occupation of Sadr City, they seem to be starting from the top and working their way down. Sadr City is a blighted urban nightmare currently, but these kinds of attacks on collaborators could rapidly help transform it into a series of armed, barricaded camps... just like much of the rest of the city.
Alternately, the people responsible could actually be Sadrists, with Sadr pretending to cooperate, while making the occupation of Sadr City considerably more costly.
Lastly, we could potentially be talking about actual Iranian agents who are doing these things, or perhaps Sunni who are trying to get the Shi'a and the U.S. fighting... but I don't think this is either as likely, or as big of a threat. Outsiders are easily isolated and killed. The biggest threat is the enemy within.
Meanwhile, there's a bit of a hydra effect going on outside of Baghdad, where people who appear to be Sunni militants who have fled Baghdad before the surge are joining with local Sunni to cause quite a ruckus. Chop a head off the monster in Baghdad, two more pop up elsewhere.
While there is talk of lower Iraqi civilian casualties in Baghdad, and while the beginning of "surge" operations started out fairly peaceful, with a lot of "wait and see" attitudes prevailing from all sides, U.S. casualties seem to be creeping up a bit over the last week, primarily due to increasingly large, sophisticated, and well-orchestrated IED attacks. If these days of four and five U.S. soldiers getting killed keep happening, expect the media to stop focusing on the message being put forward by the military and Iraqi government about how "the surge" seems to be working. Instead, expect the focus to shift back towards how "the surge" is accomplishing the one thing that critics of the operation were most concerned about -- more dead U.S. troops.
*UPDATE* : Looks like my initial suspicions on the people behind the attack are most likely to be correct.
AP is reporting that a local Mahdi Army commander suspects the attack was caused by a splinter group:
"...a local commander said suspicion fell on a group of disaffected militiamen who are angry about the deal. "This is a faction that enjoys some weight," the Mahdi Army commander said, speaking on condition of anonymity because of the sensitivity of the subject. He said the attack has created tension within the ranks of the militia and renewed a debate on the merits of allowing the Americans to operate in Sadr City without resistance..."
So, it's not some marginalized splinter group, but actually a fairly significant group which threatens to drag the rest of the Mahdi Army forces to their side, or at least to the side of being openly opposed to U.S. forces, because of the instability they cause and the attacks they encourage inside of Sadr City.