I was researching botulism and tetanus, as you do, when I came across this neat anecdote about the U.S.'s attempt to use botulism as a biological weapon during WWII. This is shortened from the original, to just the interesting bits:

During World War II, the US Office of Strategic Services (OSS) developed a plan for Chinese prostitutes to assassinate high-ranking Japanese officers with whom they sometimes consorted in occupied Chinese cities. Concealing traditional weapons on the women at the appropriate time would obviously be difficult. Therefore, under the direction of Stanley Lovell, the OSS prepared gelatin capsules "less than the size of the head of a common pin" containing a lethal dose of botulinum toxin. Wetted, a capsule could be stuck behind the ear or in scalp hair, later to be detached and slipped into the officer's food or drink. The OSS recognized that the normal background of botulism cases would deflect suspicion from the women.

The capsules were shipped to Chunking, China. The Navy detachment there, taking nothing for granted, tested the capsules on stray donkeys. The donkeys lived. Lovell was informed that the capsules were faulty, and the project was abandoned. Much later, Lovell learned of the donkey test with, one imagines, some consternation, since "donkeys are one of the few living creatures immune to botulism."

From: JAMA Vol. 285, No. 21, June 6, 2001

Now that you're excited about botulism, here's a quick refresher on what it is and why it's so bad for you:

The thing that makes you sick - the botulinum toxin - is a protein made by a bacteria. The toxin causes paralysis, usually starting in the face, spreading to the limbs, and possibly to your respiratory system. Not fun. 

Why did they want to use it to kill people?

Easy! Because botulinum toxin is "the most acutely toxic substance known." Wikipedia tells us that 4 kilos, or just about 9 pounds, of the stuff, spread all around the world, would be enough to kill every single human alive.

Does this mean I shouldn't can things? 

Everyone's always afraid of getting botulism from canned food, but really, as long as you're safe and don't can low-acid foods, it shouldn't be much of a problem. In the United States, there are only about 145 cases of botulism each year, and only about 15% of those are foodborne. In fact, using the CDC's fancy foodborne illness database, I only came across one case of botulism in 2009, and it was from home canned tuna. The great majority of the cases are in infants (don't feed babies honey!), and most of the rest are "wound botulism," often caused by injections of black-tar heroin. 

So, yeah, don't do heroin, don't eat home canned fish, and don't give babies honey. 

Bonus fun fact: The name botulism comes from the Latin "botulus," which means sausage. It's so called because it was noted, way back in the 19th century, that botulism often resulted from eating meat products, like sausage. 

Update: I take back, kinda, what I said about the etymology, above. In this except, real live academics argue about whether botulism was called sausage because that was one of the main reasons people got sick, or because the bacteria itself is rod-shaped, and looks like a sausage. No one can agree!