This caught my eye since we're discussing air travel in another thread. Personally, I never leave the US without my passport but I guess some people do...

http://travel.state.gov/travel/cbpmc/cbpmc_2223.html

Beginning January 23, 2007, ALL persons, including U.S. citizens, traveling by air between the United States and Canada, Mexico, Central and South America, the Caribbean, and Bermuda will be required to present a valid passport, Air NEXUS card, or U.S. Coast Guard Merchant Mariner Document, or an Alien Registration Card, Form I-551, if applicable.

As early as January 1, 2008, ALL persons, including U.S. citizens, traveling between the U.S. and Canada, Mexico, Central and South America, the Caribbean, and Bermuda by land or sea (including ferries), may be required to present a valid passport or other documents as determined by the Department of Homeland Security.

Boyd, good heads-up. I've been traveling overseas a bit in the past few months (and decades) and I'm really surprised how many U.S. citizens are unaware of this. Gotta say, however, we have always used our passports anywhere, domestic or to those countries who in the recent past have not required them, and some of our very sophisticad travel friends are surprised we used them when we didn't "need" to. It was a good habit to get into and I have no sympathy for those who now might be caught up short.

Sandy