I agree that the Zarqawi letter is unlikely to be real. The Associated Press has the entire text here, and after reading it, I’d put the chances of it being genuine as high as 5%.
But given this section, it doesn’t sound like anything the U.S. would produce:
The question remains, how to draw the Americans into fighting a war against Iran? It is not known whether American is serious in its animosity towards Iran, because of the big support Iran is offering to America in its war in Afghanistan and in Iraq. Hence, it is necessary first to exaggerate the Iranian danger and to convince America and the west in general, of the real danger coming from Iran, and this would be done by the following:
1. By disseminating threatening messages against American interests and the American people and attribute them to a Shi’a Iranian side.
2. By executing operations of kidnapping hostages and implicating the Shi’a Iranian side.
3. By advertising that Iran has chemical and nuclear weapons and is threatening the west with these weapons.
4. By executing exploding operations in the west and accusing Iran by planting Iranian Shi’a fingerprints and evidence.
5. By declaring the existence of a relationship between Iran and terrorist groups (as termed by the Americans).
6. By disseminating bogus messages about confessions showing that Iran is in possession of weapons of mass destruction or that there are attempts by the Iranian intelligence to undertake terrorist operations in America and the west and against western interests.
Instead, it seems transparently to be the work of the Iraqi government, perhaps with an assist from their Iranian friends.
I think there’s a larger lesson here, too. The uranium-from-Niger forgeries were so blatantly stupid you’d assume they couldn’t be the work of the U.S. or Italian intelligence services. But generally speaking, subtlety and competence are not the strong suit of people who do this kind of thing. If they were subtle and competent they’d be in another line of work.