Excuse me if this repeats something that has been discussed already, but I haven't seen it.
FYI, anyone who plans to return to the US directly from Central or South America after Dec 31, 2005 will need a US passport "or other accepted document." I suppose this is a homeland security regulation. The same goes into effect for Mexico and Canada after Dec 31, 2006. They say that now (from August to November) is a good time to apply for a passport because the backlog is less.
For further information, our friends at the Department of State have prepared: <http://travel.state.gov/travel/cbpmc/cbpmc_2225.html>
I have no idea if these new requirements cover such things like driving across the border with a hundred pounds of explosives hidden under the floorboards of the truck bed, etc. ;-)
Edward Sevcik University of Texas School of Information Austin, Texas
