According to Wikipedia, the "Corruption Perceptions Index (CPI) [orders] the countries of the world according to 'the degree to which corruption is perceived to exist among public officials and politicians'. The organization defines corruption as 'the abuse of public office for private gain.'"
If the CPI is based on perception, then whose perception is it referring to? According to the Wikipedia article, "'In the past, the experts surveyed in the CPI sources were often business people from industrialized countries; the viewpoint of less developed countries was underrepresented. This has changed over time, giving increasingly voice to respondents from emerging market economies.'" So basically the CPI is measuring corruption based on viewpoints from businesspeople.
Corruption by its nature is hidden. Some corruption is overt, but different people respond differently to corruption. For example, in many industrialized countries rich people are more successful in the legal system because they can afford to pay the legal fees. Some consider this just an unfortunate reality and that's it. Others consider it corruption.
Furthermore, most people judge countries by the people they recognize on the news who represent that country. For example, your average person might associate George Bush with America or Steve Irwin with Australia. To what extent does the attractiveness of the representative make a difference in corruption perceptions? Dr Gordon Patzer says the following: "Good-looking men and women are generally judged to be more talented, kind, honest and intelligent than their less attractive counterparts." If you ask a businessperson whether they think America or Britain is more corrupt, he might think of America and then think of a beautiful woman like Reese Witherspoon and through hardwiring will think that Reese and therefore America is not corrupt. However, when he thinks about Britain he might think of Prince Charles.
The reason why I am talking about this is because I have noticed some people who look at the Corruption Perceptions Index and then say, "Well, Country X has a high CPI and therefore that country must be very corrupt." Who is to say that the index itself is not corrupt?