There are links to a lot of information here, beginning at Michelle Malkin, and branching out.

Based on the sophisticated nature of the raid, as well as the response, or cryptic non-responses, from multiple military and intelligence sources, this raid appears to have been directed and executed by the Qods Force branch of the Iranian Republican Guard Corps. My sources agreed this is far too sophisticated an operation for the Mahdi Army or Badr Corps, while al-Qaeda in Iraq would have a difficult time mounting such an operation in the Shia south. “The Karbala Government Center raid the other day was a little too professional for JAM [Jaish al-Mahdi, or the Mahdi Army],” according to a military source.
This raid required specific intelligence, in depth training for the agents to pass as American troops, resources to provide for weapons, vehicles, uniforms, identification, radios and other items needed to successfully carry out the mission. Hezbollah’s Imad Mugniyah executed a similar attack against Israeli forces on the Lebanese border, which initiated the Hezbollah-Israeli war during the summer of 2006...

The Karbala raid makes sense in light of the U.S. raids on the Iranian diplomatic missions in Baghdad and Irbil, where Iranian Qods Force agents were captured, along with documentation that divulged Iran's involvement with and support of Shia death squads, the Sunni insurgent, and al-Qaeda in Iraq and Ansar al-Sunnah. Five Iranians from the Irbil raid are still in U.S. custody, and captured U.S. soldiers would provide for excellent bargaining chips

One interesting piece of information, Coalition troops have been given the go-ahead to kill or capture Iranian agents in Iraq and/or Afghanistan, specifically after this attack.

It might be possible that an 'accidental' gun battle between US soldiers and Iranian security agents trying to 'infiltrate' Iraq, force Americans to enter Iranian territories!