This might be fun to incorporate into a story, since it's a new program:
The U.S. government created passport cards (vs. standard passport books) that will fit in a wallet and can be used for land and sea border crossings with Mexico, Canada, Bermuda and Caribbean. It cannot be used for air travel.
The idea is in response to the new law that requires a passport to re-enter the country. It's a smaller and more convenient passport (wallet sized card) that is less expensive for those who just visit our neighboring countries.
Applications for passport cards will be available February 1, 2008 and they will be issued in spring 2008.
A bit of trivia...do you know which agency issues passports?
More information is here.
Answer to trivia question: Department of State