My betting is Iraq to win, Al Qaeda to place. If Al Qaeda did it, they probably got the material from Iraq or possibly made their own.
1. The anthrax is fairly sophisticated, with steps taken to make is disperse in an aerosol. The feds were saying it would take a PhD biologist to pull that off, and I don't see some nutcase militia having the smarts to do that. Renting Ryder trucks they can handle, doing the years of tinkering to get this to work, no. (Yes, I know the militia guy arrested a few years ago, but don't think he'd typical.)
2. The anthrax appeared shortly after the 9/11 attacks, with postmarks a week or two after the attack. The person who sent the letters had the anthrax ready to go beforehand. Maybe this was just a convienient trigger point for their pre-existing plan, but I doubt it. If they had the anthrax ready to go they probably would have already used it against abortion clinics or whatever.
3. Postmarks in Trenton, NJ, a hotbed of Islamic extremism.
4. The attacker appears to be east coast oriented. People of this sort tend to attack the things they see and obsess over every day. So far as we know the letters weren't sent to the sorts of people someone from the midwest or far west would see on the tube every day, while the targets hit so far, especially Pataki, suggest someone in the NY media market. There aren't as many violent militias around there.
5. The handwriting and messages in the letters are consistent with a foreigner. Maybe it's someone trying to appear to be a foreigner, but on the other hand, maybe there is not fakeout.
6. The anthrax doesn't appear to be engineered to be antibiotic resistant. That would probably eliminate the Russians or escaped Russian weapons. OTOH, Iraq would probably not place as much importance on antibiotic-resistant strains if it thought the Iranians had lousy medical care anyway.