This will primarily effect cruise passengers, and those traveling between the U.S. and Canada or Mexico. Currently U.S. citizens can travel via cruise ships, or drive accross the borders with simply a birth certificate and photo ID. However, after June 1st everyone will be required to have a valid Passport, or the new Passport Card.
If you need to get a new U.S. Passport or renew yours in 2009, make sure to allow plenty of time, as it sounds like the Passport Agencies are getting pretty backed up.
See the U.S. Department of State's latest Press Release on the Western Hemisphere Travel Initiative.