You make some good points, but my guiding principle is that (as Mark Twain said): "... if you find yourself agreeing with the majority (the mob) you'd better step back and take another look.. ". (Or something to that effect).
I agree that suspicion for the mailings points to the usual suspects. But it's suspicion, not evidence.
And as far as basing logic on past trends goes, I bet you thought Mid-Easterners attacked the OK building. But that's alright, practically everybody else thought so too.
If you are as immune to Anthrax as you are to logic, you have nothing to worry about.