Do you need a passport to go on a cruise to Mexico in June 2009?

On cruises that disembark and port from the same U.S city, passengers only need proof of citizenship ( a birth certificate works in this case). You also don't have to have a passport to enter Canada and U.S customs will not stop U.S citizens from re-entering the the U.S.A but they have stated that they will make the process much more difficult.