^^ Edit.
Oops. It turns out that the US did sign the protocol, but never ratified it.
“The United States signed the Protocol on November 12, 1998. However, the
Clinton Administration did not submit the Protocol to the Senate for advice and
consent, acknowledging that one condition outlined by S.Res. 98, passed in mid-1997
— meaningful participation by developing countries in binding commitments
limiting greenhouse gases — had not been met. In late March 2001, the Bush
Administration rejected the Kyoto Protocol. The United States continues to attend
the annual conferences of the parties (COPs) to the UNFCCC, but does not
participate in Kyoto Protocol-related negotiations. In February, 2002, President Bush
announced a U.S. policy for climate change that will rely on domestic, voluntary
actions to reduce the “greenhouse gas intensity” (ratio of emissions to economic
output) of the U.S. economy by 18% over the next 10 years.”
~ Page 2, Paragraph 3, Global Climate Change: The Kyoto Protocol (2004 Update), Library of Congress Congressional Research Service Report, Susan R. Fletcher, Specialist in Environmental Policy Resources, Science, and Industry Division, United States Department of State.