"Too early to tell," was the alleged famed response from Chinese Prime Minister Chou En-Lai in 1972 to U.S. Secretary of State Henry Kissinger's question on the effect of the 1789 French Revolution. It's possible that some 183 years from now statesmen will give a similar answer about the long term effect of the November 28, 2010, publication by Wikileaks of some 250,000 confidential U.S. diplomatic cables.
The fact that secret discussions between foreign leaders and U.S. diplomats ended up on the front pages of publications across the globe by necessity forces foreign governments to reassess how they do business with the U.S. When current Lebanese Prime Minister Saad Hariri told U.S. diplomats back in 2006 that "Iraq was unnecessary, Iran is necessary," he no doubt did so assuming that his comments would be kept confidential. Given the power of Iranian proxy Hezbollah in Lebanon, his publicly urging action against Iran is dangerous.
While the State Department is not directly to blame for a rogue official leaking documents, in the eyes of other countries the U.S. has shown a worrying inability to protect private conversations. There's no certainty that this won't be repeated, and so tongues will naturally be more guarded in future meetings. Countries cannot and will not stop dealing with the U.S. – it is the world's superpower and other countries must work with it – but they'll likely seek alternative messengers to local U.S. diplomats where possible. Either they'll go directly to Washington with their messages, or they'll turn to trusted third parties to deliver them.
Beyond the lack of trust created by the leaks, there's the personal offense caused to foreign leaders by comments made about them in the memos. French President Nicolas Sarkozy was described in a cable as an "Emperor with no clothes," Russian President Dmitry Medvedev "plays Robin to Putin's Batman," German Chancellor Angela Merkel "avoids risk, not very creative," Turkish Prime Minister Erdagon has "an authoritarian loner streak" and a "distrust of women," the mental health of Argentine President Cristina Kirchner was questioned, and Italian Prime Minister Silvio Berlusconi was described as "feckless, vain, and ineffective as a modern European leader."