Who are the insurgents?

I don't know who these writers are, nor do I know how reliable their information is. The only name I recognize on the Home Page is Daniel Pipes, not someone I consider a person who supports democracy after he wrote his article in support of the internment of the Japnese Americans in WW II. That said, what the article says fits what I know so far about the insurgents and does not have any glaring inconsistencies with other information out there. This is from Middle East Intelligence Bulletin.

They report the existance of a group calling itself Ansar al-Sunna (Defenders of the Tradition).

Ansar al-Sunna is not purely a homegrown terrorist operation. Many of its recruits appear to be infiltrators from beyond the borders of Iraq. It has taken root in Iraq, and especially the area around Mosul, both because of Coalition failure to secure the Syrian and Iranian borders with Iraq and because of a reconciliation policy which mitigates pressure upon planning and organization. Iranian and Syrian officials are likely involved in facilitating the group member's infiltration to Iraq given the evidence of visas and safe houses. The wide variety of weapons the group appears to have at its disposal, as well as the level of its media sophistication, suggests that the group may have access to outside financial resources. The group's militant Salafi ideology suggests that co-option may not mitigate the Ansar al-Sunna threat in Iraq. At the same time, the group's failure to establish itself in southern Iraq suggests that the group's area of operations inside Iraq will be contained to Baghdad, the Sunni triangle, and northern Iraq. Nevertheless, Ansar al-Sunna's clear attempts to expand outside Iraq necessitate further military pressure inside Iraq to prevent the northern Iraqi city of Mosul from transforming itself into a safe-haven for terrorist planning that could threaten not only Coalition forces and Iraq's interim government, but also Western targets beyond Iraq's borders.

Note that this is a northern Iraq operation. Problems in southern Iraq will have a different source, possibly with Iranian support.