It is quite likely the sender of the anthrax letters has been vaccinated.
As to Rush's proposed Iraqi connection. I think Saddam Hussein understands very well that an Iraqi sponsored biological weapons attack on the U.S. would be the end of his regime, most likely in a hail of fuel-air weapons, though possibly with nukes thrown in.
On the other hand, there are almost certainly elements in the Iraqi establishment who are more sympathetic to al Qaeda's brand of Islam than to the secularism of the Baathist movement. Rogue Islamists in Iraq would fit with
- Availiability of good quality, though not properly "weaponized" anthrax to al Qaeda.
- A meeting between Atta and an Iraqi operative in Prague.
- The Chechen plot, lead by an Iraqi, to assassinate Putin. (Absolute insanity, from Saddam Hussein's point of view--Russia is one of the few countries willing to trade with Iraq.)
The Islamists would even welcome an American attack on Iraq (and particularly one on shaky evidence) as it would bolster their propaganda campaign with the Muslim masses, and would remove a strong secularist Arab government all in one stroke.
I just think Hussein supplied it, and that he thought 3rd party plausible deniability was enough. He thinks we won't dare attack him -- see the book link posted above