Starting June first, if you leave the country, you're going to need a passport to get back in even if you're not flying.
The new rule covers all U.S. Citizens 16 and over.
This is in addition to the January 2007 law which required all air and sea travelers to have a passport.
Now all travelers have to have them.
If you don't want to shell out a hundred bucks for a passport, you can get a passport card which will let you cross the border by land only.
For more information about the new law and how you can get your passport, just log onto travel.state.gov.