CHINA, AS EVERYONE KNOWS, IS A BIG FORCE IN THE extraordinary boom in commodities. Its voracious appetite for everything from corn and wheat to copper and oil has helped push up U.S. commodities prices by some 50% over the past 12 months. But China is by no means the whole story. Speculators -- including small investors -- are also playing a huge role. Thanks to the proliferation of mutual funds and exchange-traded funds tied to commodities indexes, speculative buying has gone way beyond anything the domestic commodities markets have ever seen. By one estimate, index funds right now account for...