The US Department of State has sent out a reminder that all travelers will once more be required to present valid passports for re-entry into the US as of 1 October 2007, in compliance with the Western Hemisphere Travel Initiative (WHTI) aimed at strengthening border security. The WHTI passport requirement for air travel went into effect on 23 January 2007, but the US government temporarily instituted a waiver for US travelers who could show proof of having applied for the passport.
Note that this is a requirement of the US government, not the Dominican government.