I am a TA and have been instructed to let all my clients know the following:
Jan 1, 2006 a passport will be required to re-enter the US from travel in the caribbean.
Jan 1, 2007 a passport will be required for re-entry into the US - VIA air travel from Caribbean/Mexico/Canada.
Jan 1, 2008 a passport will be required for re-entry into the US via border crossings from Canada and Mexico.
We have been receiving e-mails every day form different passport and government agencies and the dates and areas of travel are all slightly different right now until the regulation is passed, so all I can say is...
GET A PASSPORT! I cannot stress enough how much easier it is to travel, internationally AND domestically!