The rules have recently changed for US citizens traveling abroad. As of June 1st, 2009:
US citizens 16 years or older will need a valid passport, a valid passport card, or other documentation approved by Homeland Security.
Children under 16 may currently travel with a birth certificate (original state issued or a certified true copy).
Foreign citizens will need a passport, plus any other documentation mandated by the US or Bahamian governments.
If you have ANY questions, call your local Customs and Immigrations office at 1-800-375-5283 or go online to the US Department of State website. To apply for a US passport click here.