As Anna Moore Lappé and Willie Nelson (yeah, that Willie Nelson) write in an essay just published on Huffington Post:
Today, three companies process more than 70 percent of all U.S. beef: Tyson, Cargill, and JBS. More than 90 percent of soybean seeds and 80 percent of corn seeds used in the United States are sold by just one company: Monsanto. Four companies are responsible for up to 90 percent of the global trade in grain. And one in four food dollars is spent at Walmart.
More reading: Vandana Shiva has an even more interesting summary of the global effects of treating seeds like intellectual property. A couple of filmmakers have summed up the problem in a two-minute video illustrating a Michael Pollan excerpt. And the grand-daddy -- and most compelling -- of all these Occupy Our Food Supply essays was written by Tom Philpott back in October.
If you want to participate in the day of action, you can Occupy Cargill (well, call the company up to lodge your protest against its policies). San Francisco and Oakland are hosting a number of events, including workshops and a seed exchange at 101 Market (BofA's headquarters), a work day at Little City Gardens, and possibly a march on Safeway.