After January 2007 all American citizens traveling by air to the United States will have to present a US passport.
A passport is an easily recognized travel document authorizing travel and identifying you. You need a passport to enter a foreign country and return to the US. To get a US passport through the government, not commercial passport application agencies contact: http://travel.state.gov/passport/get/first/first_830.html
For those who already have a passport, make a photo copy of the passport, and keep it and the original passport in a safe place. Do not give your passport to anyone outside the hospital.