I've wondered from time to time if he was supposed to follow the attack in NY or D.C. with a crop duster spreading the anthrax. IIRC, the FBI found info about crop dusting on his computer. Such a follow-up would be perfectly in keeping with their enjoyment of targeting rescue crews with a secondary explosion. If that scenario were correct, then the letters were a "plan B" that had to go into effect after he was arrested.
The only problem I see with that scenario is that I'd have expected them to hit both NY and DC, which probably would have required another pilot, and we've not heard of anyone else being preempted.
I was thinking airliner- do you know of any articles which say what Moussaoui had studied or asked to train on in flight school, or how far he got in his studies? Atta homed right in on the correct simulator.