Well, after all of the people who got sick, the several people who died, the hundreds of thousands spent on investigating the outbreak, and the FDA says that the culprit was...

Well...

...they still don't really know.

Because the contamination occurred before the start of the investigation, and because of the many ways that E.coli O157:H7 can be transferred -- including animals, humans, and water -- the precise means by which the bacteria spread to the spinach remain unknown.

That's not to say that they don't have their suspicions. They were able to identify the environmental risk factors and the areas that were most likely involved in the outbreak. "Contaminated irrigation water, uncomposted manure used as fertilizer, the presence of wildlife and livestock and the hygiene of the workers handling the crops all might have served to transport the bacteria", they said.

However - Fresh Express has seemingly come to their own conclusions on the causes and is refusing to buy lettuce and spinach from farmers who don't stop using compost and recycled water.

This action by Fresh Express is both welcome and needed. At the very least, it will provide an interesting comparison against produce companies who do not have similar requirements.

UPDATE: As Jack alluded to in the comments, compost is actually preferable to using chemical fertilizers in produce use, something that slipped my mind completely. Personally, I believe that the recycled sewage water played the larger role, and was focused more on that than the compost issue.