Nope, it's not the U.S. Japan, Germany and France have higher penetration rates, at much higher speeds. Why is that?
Well, Paul Krugman of the New York Times blames it on the "digital robber barons"  who control broadband:
What happened to America’s Internet lead? Bad policy. Specifically, the United States made the same mistake in Internet policy that California made in energy policy: it forgot — or was persuaded by special interests to ignore — the reality that sometimes you can’t have effective market competition without effective regulation.
You see, the world may look flat once you’re in cyberspace — but to get there you need to go through a narrow passageway, down your phone line or down your TV cable. And if the companies controlling these passageways can behave like the robber barons of yore, levying whatever tolls they like on those who pass by, commerce suffers.
America’s Internet flourished in the dial-up era because federal regulators didn’t let that happen — they forced local phone companies to act as common carriers, allowing competing service providers to use their lines. Clinton administration officials, including Al Gore and Reed Hundt, the chairman of the Federal Communications Commission, tried to ensure that this open competition would continue — but the telecommunications giants sabotaged their efforts, while The Wall Street Journal’s editorial page ridiculed them as people with the minds of French bureaucrats.
And when the Bush administration put Michael Powell in charge of the F.C.C., the digital robber barons were basically set free to do whatever they liked. As a result, there’s little competition in U.S. broadband — if you’re lucky, you have a choice between the services offered by the local cable monopoly and the local phone monopoly. The price is high and the service is poor, but there’s nowhere else to go.