I think Left-Wing Communist groups would be more likely to send physical poison thru the mail since they have been sending psychological poison thru the mail, over the airways and in the public schools for a 100 years.
The worst part about this mail terror, is that they will never catch the culprits. They only caught the Una-Bomber because he screwed up and published that wacky manifesto. My guess is, we'll never know who's sending this stuff.
And such a thing is indeed a plausible way to "further the revolution" that both right- and left-wing fringe groups claim they want. And of course, Saddam would be more than happy to deal with people who are in the market for somthing that futhers his cause.
The strategy of anthrax for a domestic revolutionary group is simple: anthrax begets strict controls, which turn people against the authorities. This makes it easier to peel off some of the more volatile among the otherwise law-abiding citizens. (We have plenty here at FR, and I'd venture to guess that the Democratic Underground has its own version.)
Then it's on to something different, with an eye toward stricter government controls, and so the cycle goes.
The tactics themselves are Mao's. I can easily see how the people using anthrax could be domestic: right-wing, left-wing, even both. It wouldn't surprise me in the least.