Their correspondence led to a debate at Berkeley in February of 2007. The video is really long (around 2 hours) and covers many of the same points as the correspondence, but it's worth watching if you're interested in the environmental and social issues around food production and have some free time.
The letters and the video all bring up a number of topics we've discussed in class, like whether businesses that aim to be socially responsible are just opening themselves up to extra scrutiny and how good is good enough when you're an ethical brand.