Felix Salmon at Portfolio points out an arbitrage opportunity in Berkshire:
"In my opinion," says Warren Buffett, "when the B is at a discount of more than say, 2%, it offers a better buy than the A." Right now the discount is a whopping 7%. So if you want to take Buffett's advice, start buying up B shares. If, that is, you want to buy his stock in the first place. Which is a different matter entirely.