Antrax is a poor choice for a weapon of mass distruction. It has been used well in this situation as a weapon of mass hysteria.
If terrorists choose to kill a great number of people (and I'm not conviced that that is their goal), they will use an infectious disease -- like Small Pox. Small Pox is a CDC nightmare.
Small Pox vaccinations stopped many years ago. Most young people would be entirely unprotected. The medical community is unsure whether the vaccinated masses would be resistant to newer strains or even to the original strain. Not having the disease around for people to fight off has brought the efficacy of the vaccination into question.
Distribution of Small Pox would be easily accomplished, and would produce an pandemic.
But is that really their goal? No - their goal is TERROR and related disruption. Why go for mass destruction when the fear of it is so much easier to invoke? It's not so much the 6000 WTC dead that excites the terrorists, it's the 100,000 layoffs, 20% stock market drop, 3-hour waits at airline checkpoints, confiscated nailcutters, national fear, etc. etc. etc. They may want us dead, but the subsequent national terror invoked by a mere envelope of talcum powder is ultimately more useful. They want to see the most powerful nation on earth cower in fear when someone says "Boo!"