From the New York Times (link):
Taiwan’s president, Chen Shui-bian, delayed an overseas trip at the last minute on Wednesday after the United States denied him permission to stop overnight on American soil, a diplomatic setback for the independence-leaning Taiwanese leader.
The rebuke suggests that the Bush administration remains displeased by Mr. Chen’s move in February to abolish a government agency set up to study reunification with mainland China. The United States is also eager to avoid antagonizing China after some embarrassing incidents during a visit by its president, Hu Jintao, late last month.
Bush administration officials need China’s cooperation to curtail Iran’s nuclear program, and its willingness to follow the American lead on such issues often depends on whether it perceives the United States as helping to check Taiwan’s independence drive.