Previously you didn't need a passport to travel to the Dominican Republic. You could have entered the country on a birth cetificate, license, or some other form of ID, but the U.S. Senate has passed an amendment which requires all Americans to have a passport to enter any Caribbean nation, beginning on January 8, 2007. The amendment does exclude cruise ship vacations, but will eventually encompass this form of tourism also.

UPDATE: 8 June 2007
The Homeland Security announced that US citizens traveling to Canada, Mexico, the Caribbean and Bermuda who have applied for but not yet received passports can enter and depart from the US by air through 30 September 2007. Travelers will be required to present government-issued photo identification and a Department of State official proof of application for a passport. The temporary change in procedures is to facilitate summer travel. It usually takes 30-45 days to receive a passport after applying. It is estimated that only 25% of Americans have passports, which meant the new requirement was a major inconvenience for millions of summer travelers that complained to their legislators.
The announcement is good news for the Dominican Republic tourism industry that had been affected by the passport rule.
See http://www.dhs.gov/index.shtm