The US dropped to 24th (of 178) for its worst showing ever on the Corruption Perceptions Index (CPI). Previously, the worst performance was in 2006-07 when the US came in 20th. The best the US has ever placed was in 2001-02 when it was 16th.
Since the CPI has grown in terms of number of countries, perhaps a better way to look at a country is by way of the absolute score. Even here, though, the US has seen some degradation over the years. In 2001, the score was 7.6 (out of 10 - high numbers are better) while the 2010 score was just 7.1.
Still, the US is in the top 13% of the world. While there are a few seemingly counter-intuitive results (as my headline suggests), most of the least corrupt countries are in the OECD. The most corrupt places on the planet continue to be concentrated in Africa (Sudan, Chad, Burundi, Somalia) and Central Asia (Turkmenistan, Uzbekistan). Both Afghanistan (with a 1.4 out of 10) and Iraq (with a 1.5) are listed among the most corrupt countries on earth.
According to TI, "the surveys and assessments used to compile the index include questions relating to bribery of public officials, kickbacks in public procurement, embezzlement of public funds, and questions that probe the strength and effectiveness of public sector anti-corruption efforts." A very complete run-down of their methods and sources is available on their website.