Here's an interesting article by Ian Mather on Scotsman.com about the insurgent groups in Iraq.

"There is no centre of gravity, no leadership, no hierarchy; they are more a constellation than an organisation," says Bruce Hoffman, a terrorism expert at the Rand Corporation of the new-look insurgency. "They have adopted a structure that assures their longevity." As a result, even killing or capturing al-Zarqawi would not end the rebellion.